210 F.3d 741 (7th Cir. 2000)
UNITED STATES OF AMERICA,    Plaintiff-Appellee,v.MICHAEL A. ROE,    Defendant-Appellant.
Nos. 99-2541, 99-2758
In the  United States Court of Appeals  For the Seventh Circuit
Argued January 21, 2000Decided April 11, 2000

Appeals from the United States District Court  for the Western District of Wisconsin.  No. 98 C 3946--Barbara B. Crabb, Judge. [Copyrighted Material Omitted]
Before BAUER, RIPPLE and KANNE, Circuit Judges.
RIPPLE, Circuit Judge.


1
A jury found Michael Roe  guilty of conspiring to distribute cocaine in  violation of 21 U.S.C. sec. 846. The district  court held Mr. Roe responsible for conspiring to  distribute 623 grams of cocaine and based his  sentence on that amount. On this appeal, Mr. Roe  seeks the reversal of his conviction. He contends  that the district court improperly allowed the  Government to enter evidence of his prior cocaine  conviction and to make improper statements during  closing argument. Mr. Roe also challenges the  district court's sentencing decision. He contends  that the court improperly determined the amount  of cocaine attributable to him. For the reasons  set forth in the following opinion, we affirm the  judgment of the district court.


2
* BACKGROUND


3
Michael Roe was convicted of conspiring with  Timothy Weger to distribute cocaine. There is no  dispute that Weger was involved in the  distribution of cocaine; he pleaded guilty to  cocaine conspiracy and then testified against Mr.  Roe at trial. A Madison, Wisconsin, police  officer testified that he had found drug  paraphernalia in Weger's trash, and more  paraphernalia was found inside Weger's home.  Further, there is no dispute that Mr. Roe and  Weger engaged in cocaine transactions with each  other. Mr. Roe's defense at trial was that he was  not a co-conspirator of Weger's, but instead was  merely a customer. Weger testified that some of  the paraphernalia at his house belonged to Mr.  Roe.


4
At trial, the Government questioned Mr. Roe  about his prior cocaine-related conviction. In  1989, Mr. Roe had pleaded guilty to possession of  cocaine with intent to distribute. In addition to  asking Mr. Roe about his prior conviction, the  Government referred to the issue in its closing  argument. After trial, the district court issued  an opinion clarifying its basis for allowing this  evidence under Federal Rules of Evidence 404(b)  and 609. The court noted that Mr. Roe had put at  issue the absence of cocaine paraphernalia at his  own house. Therefore, this evidence helped  explain why he would not keep such paraphernalia  there but might instead keep it at Weger's house.


5
The Government made two comments during closing  argument that Mr. Roe claims misstated the law.  The first comment, to which Mr. Roe did not  object at trial, was a statement of the law of  conspiracy: "Basically what we have to do is  prove to you that there was an agreement. That  there was something going on between those two  men, an agreement." R.98 at 57. The second  statement was as follows:


6
Last point. Mr. Roe got up there and did a big  deal about basically what was a role reversal.  "I'm not the source. I'm the customer. Tim's the  source." It's total poppycock, but believe it if  you will. He's still guilty. Just reversing the  roles doesn't get you out of the conspiracy.  Whether one guy is a source and one guy is the  customer who's selling, or the other way around,  it's still, you're in a conspiracy.


7
R.98 at 110. Mr. Roe did make a contemporaneous  objection to this statement, and the district  court told the jury that only the court could  instruct the jury as to the elements of a  conspiracy charge.


8
At sentencing, the district court attributed 623  grams of cocaine to Mr. Roe. The district court  had relied on several pieces of evidence in  arriving at this amount. Timothy Weger had  testified about the amount of cocaine he saw Mr.  Roe handle. Tina Weger, Timothy Weger's wife, had  testified about a pile of cocaine she saw Mr. Roe  weighing in the Weger basement and had made hand  motions to demonstrate the size of the pile.  Further, the Government had introduced evidence  that the cocaine it took from the Weger house was  45% pure and evidence of empty bottles found in  the house that had contained 10 ounces of  inositol, a cocaine cutting agent. From the  purity of the cocaine and the amount of inositol  used, the district court determined that over a  pound of cocaine was involved in the conspiracy.

II
DISCUSSION
A.

9
Mr. Roe's first contention is that the district  court improperly admitted evidence of his 1989  conviction for conspiracy to distribute cocaine.  Federal Rule of Evidence 404(b) allows the  admission of evidence of other crimes, wrongs, or  acts under certain circumstances.1 For evidence to be admissible under Rule 404(b), it must meet  four requirements:


10
(1)  the evidence is directed toward establishing  a matter in issue other than the defendant's  propensity to commit the crime charged; (2) the  evidence shows that the other act is similar  enough and close enough in time to be relevant to  the matter in issue; (3) the evidence is  sufficient to support a jury finding that the  defendant committed the similar act; and (4) the  probative value of the evidence is not  substantially outweighed by the danger of unfair  prejudice.


11
United States v. Asher, 178 F.3d 486, 492 (7th  Cir.), cert. denied, 120 S. Ct. 359 (1999);  United States v. Brooks, 125 F.3d 484, 499-500  (7th Cir. 1997). We review the district court's  decision to admit evidence pursuant to Rule  404(b) for an abuse of discretion. See United  States v. Robinson, 161 F.3d 463, 466 (7th Cir.  1998), cert. denied, 119 S. Ct. 1482 (1999);  United States v. Smith, 103 F.3d 600, 602 (7th  Cir. 1996).


12
Mr. Roe argues first that the district court did  not properly engage in the four-part inquiry  required, and therefore that it has abused its  discretion. See United States v. Nagib, 56 F.3d  798, 806-07 (7th Cir. 1995). It is true that the  district court's order does not discuss  explicitly all four factors. On this record,  however, we do not believe that the district  court abused its discretion. The district court's  post-trial clarifying order gives us a sufficient  basis for appellate review.

1.

13
First, we must consider whether the evidence was  directed toward establishing a matter in issue  other than Mr. Roe's propensity to commit  cocaine-related crimes. The district court, in  its post-trial order, said that the earlier  conviction showed Mr. Roe's motive for not having  drug paraphernalia at his house. It added that  the conviction also helped to explain "the  unusual fact that [Mr. Roe] did not put any trash  out at the curb in front of his residence for  collection by municipal crews." R.70 at 2. Mr.  Roe had admitted on cross-examination that his  prior conviction for cocaine distribution taught  him not to keep drug paraphernalia at his  residence.


14
We believe the district court fairly  characterized the absence of cocaine at Mr. Roe's  house as a matter at issue. As part of his case,  Mr. Roe offered testimony from Madison Police  Department Detective Rolly Squire, who said that  no drug paraphernalia was ever found at Mr. Roe's  house. Squire also testified that police officers  were instructed to search Mr. Roe's trash, but  that Mr. Roe never put out any trash. The prior  conviction was relevant in explaining Mr. Roe's  motive for not leaving any trash on the curb.

2.

15
Second, we must consider whether the acts were  similar enough and close enough in time. The  district court described the prior conviction as  being for "essentially the same conduct." R.70 at  2. Mr. Roe correctly points out that the district  court's order did not address the issue of the  time lag between the cases. However, in upholding  the exercise of discretion by district courts, we  have allowed the admission of Rule 404(b)  evidence with greater temporal gaps than the one  at bar. See United States v. Tringali, 71 F.3d  1375, 1379 (7th Cir. 1995) (1984 cocaine  trafficking conviction introduced at trial for  drug deals in 1994); United States v. Wimberly,  60 F.3d 281, 285 (7th Cir. 1995) (child  molestation conviction 13 years earlier).  Therefore, we decline to find an abuse of  discretion in the decision to admit evidence of  the 1989 conviction in a trial for offenses  allegedly committed in 1996.

3.

16
As for the third element, Mr. Roe admits that he  was convicted of a crime. His argument on this  point is that the Government did not establish  the exact facts surrounding the earlier  conviction. In this case, the fact of conviction  is all that is necessary to show that Mr. Roe had  a reason to keep drug paraphernalia out of his  house. The desire to act in a manner calculated  to avoid a second conviction for distributing  cocaine would not be dependent on the details  underlying the prior conviction. Because the  conviction itself is undisputed, we do not find  error on this ground. See Tringali, 71 F.3d at  1379 (finding third factor satisfied because  defendant was "actually convicted").

4.

17
Finally, the district court balanced, under  Federal Rule of Evidence 403, the probative value  of the evidence against its potential for unfair  prejudice. The district court acknowledged that  the similarity of the 1989 conviction to the  instant proceeding would be prejudicial, but  found that its probative value outweighed the  prejudice. "A district court's Rule 403 balancing  is afforded a special degree of deference: only  in an extreme case are appellate judges competent  to second-guess the judgment of the person on the  spot, the trial judge." United States v. Robbins,  197 F.3d 829, 843-44 (7th Cir. 1999) (quoting  United States v. Dillard, 43 F.3d 299, 305 (7th  Cir. 1994)). The district court explained the  probative value of the evidence, and we share its  view that the evidence was in fact probative. We  shall not second-guess the district court's  decision that the probative value of this  evidence outweighed any unfair prejudice to Mr.  Roe.


18
In conclusion, Mr. Roe's prior conviction  satisfied all four of the required elements under  Rule 404(b). In evaluating a district court's  decision to admit evidence under Rule 404(b), we  accord the district court's decision great  deference. See Asher, 178 F.3d at 494; United  States v. Stevenson, 942 F.2d 1111, 1117 (7th  Cir. 1991); United States v. Parkin, 917 F.2d  313, 317 (7th Cir. 1990). In this case, the  district court did not abuse its discretion in  admitting the evidence of Mr. Roe's prior  conviction.

B.

19
Mr. Roe also claims reversible error because of  two statements made by the prosecutor at closing  argument. To evaluate this claim, we first must  look at the remarks in isolation to determine if  they were improper. See United States v. Brisk,  171 F.3d 514, 524 (7th Cir.), cert. denied, 120  S. Ct. 150 (1999); United States v. Lovelace, 123  F.3d 650, 655 (7th Cir. 1997), cert. denied, 522  U.S. 1132 (1998). If they were improper, we  examine them in light of the record as a whole,  to determine whether the defendant was deprived  of a fair trial. See Brisk, 171 F.3d at 529;  Lovelace, 123 F.3d at 655. We review the district  court's decision that the defendant was not  deprived of a fair trial for an abuse of  discretion. See United States v. Miller, 199 F.3d  416, 422 (7th Cir. 1999); United States v.  Rivera, 153 F.3d 809, 814 (7th Cir. 1998). When  the defendant has failed to object  contemporaneously to the remarks, we review only  for plain error. See Rivera, 153 F.3d at 814;  United States v. Rose, 12 F.3d 1414, 1422 (7th  Cir. 1994).

1.

20
The first statement that Mr. Roe claims is  erroneous is one to which he did not object at  trial, and therefore our review is for plain  error. Mr. Roe, in his brief to this court,  claims that the prosecutor misstated the law by  telling the jury "that all the government had to  show to establish the conspiracy was '[t]hat  there was something going on between those two men.'"  Appellant's br. at 15 (quoting R.98 at 57).


21
At the outset, it is important to evaluate the  prosecutor's remark in context. The complete  statement made by the prosecutor was as follows:


22
First of all, what you've got on the ELMO there  are the elements. That's the jury instruction  that Judge Crabb is going to read to you. There's  two elements in a conspiracy. One, the government  has to prove that the conspiracy, as we charted  it in the Indictment, existed. Basically what we  have to do is prove to you that there was an  agreement. That there was something going on  between these two men, an agreement. That's the  first thing we have to do. The second thing we  have to do is show that the defendant knowingly  became a member of that conspiracy with an intent  to further it. We have to show membership or  joining in the conspiracy.


23
R.98 at 57. The prosecutor told the jury that the  first element of the conspiracy was met by an  agreement between Weger and Mr. Roe; the second  element was met by Mr. Roe's knowingly joining  the conspiracy with an intent to further its  aims. There was no misstatement of the law. We  have explained the elements of a conspiracy under  21 U.S.C. sec. 846 as follows:


24
In order to prove a conspiracy conviction, the  government must provide substantial evidence that  a conspiracy existed and that the defendant  knowingly agreed to join that conspiracy. The  government may establish each element of a  conspiracy through circumstantial evidence. With  respect to the first element, an agreement, the  government need not establish a formal agreement  to conspire.


25
United States v. Turner, 93 F.3d 276, 281-82 (7th  Cir. 1996) (citations omitted and emphasis  added). The prosecutor's statement was consistent  with the law of conspiracy as explained by this  court. Therefore, these remarks were not  improper.

2.

26
The second remark that Mr. Roe claims was  erroneous is one to which he did object at trial.  We thus review the district court's decision not  to grant a mistrial based on this comment for an  abuse of discretion. We repeat the comment made  by the prosecutor:


27
Last point. Mr. Roe got up there and did a big  deal about basically what was a role reversal.  "I'm not the source. I'm the customer. Tim's the  source." It's total poppycock, but believe it if  you will. He's still guilty. Just reversing the  roles doesn't get you out of the conspiracy.  Whether one guy is a source and one guy is the  customer who's selling, or the other way around,  it's still, you're in a conspiracy.


28
R.98 at 110. Mr. Roe argues that, because his  defense was that he was merely a customer of  Weger's, this statement, which he says is a  misstatement of the law, unfairly prejudiced him  before the jury.


29
This remark was not a misstatement of the law.  The two actors in the final sentence quoted are a  "source" and "the customer who's selling." Fairly  read in its entirety, the prosecutor's statements  did not imply that a mere customer was a member  of a conspiracy, only the customer of a source  who then re-sells to others. In effect, the  prosecutor said that, even if Mr. Roe was not  Weger's source--as Weger had testified he was--  Mr. Roe could still be found guilty of a  conspiracy if Mr. Roe purchased cocaine from  Weger for resale to others, rather than for  personal use. Although the simple act of buying  cocaine does not create a conspiracy between a  buyer and a seller, a conspiracy may be found  when the seller understood that the buyer  intended to resell the drugs to others. See  United States v. Brack, 188 F.3d 748, 760 (7th  Cir. 1999).


30
Although this remark was not improper, we also  note that the district court acted quickly to  remedy any confusion that might have arisen. When  the statement was made, the district court  responded to Mr. Roe's objection by telling the  jury that it would provide proper instructions on  the elements of conspiracy. Mr. Roe has not  argued to this court that any of the jury  instructions were erroneous. We do not believe  that any statements by the prosecutor in closing  arguments unfairly prejudiced Mr. Roe.

C.
1.

31
Mr. Roe also submits that the district court  overstated the amount of cocaine attributable to  him. The district court held Mr. Roe responsible  for 623 grams (21.8 ounces) of cocaine. Under 21  U.S.C. sec. 841(b)(1)(B)(ii)(I), the statutory  minimum sentence for distributing more than 500  grams of cocaine is 5 years, but that minimum is  increased to 10 years if the offender has a  previous conviction. Thus, when the district  court found Mr. Roe responsible for 623 grams of  cocaine, it sentenced him to 10 years'  imprisonment.


32
The district court's determination of the amount  of cocaine attributable to Mr. Roe is ultimately  a factual one that we shall review only for clear  error. See United States v. Jean, 25 F.3d 588,  598 (7th Cir. 1994). Such a review is not,  however, a toothless one. We cannot uphold a  sentence based on unreliable information. See  United States v. Humphrey, 154 F.3d 668, 671 (7th  Cir. 1998); United States v. Burke, 148 F.3d 832,  836 (7th Cir.), cert. denied, 119 S. Ct. 572  (1998). Testimony from informants who are former  drug addicts must be subject to special scrutiny.  See United States v. Beler, 20 F.3d 1428, 1432  (7th Cir. 1994). On the other hand, in our  assessment, we must remain aware of the practical  difficulties that confront the district court as  it makes this crucial determination. In  determining the amount of contraband involved,  the district court often must deal with evidence  that is stated with less than laboratory-standard  precision. Rather than rely upon the precise  tools of the scientist to ascertain the amount  involved, the district court must rely on its own  skills of critical assessment of testimonial and  circumstantial evidence. Here, our cases  demonstrate that an articulated methodology for  assessment, combined with thoroughness in the  execution of that methodology, usually signal  that the district court has made the sort of  reasoned decision that ought not be disturbed on  appeal. As we noted in United States v. Duarte,  950 F.2d 1255 (7th Cir. 1991), although the  district court's determinations need not "emulate  the precision of Newtonian physics," they also  may not be based on "nebulous eyeballing." Id. at  1265.


33
Our review of the record convinces us that the  district court well understood the issues that  needed to be resolved in determining the amount  of cocaine that ought to be attributed to Mr.  Roe. It articulated a decisional matrix for  assessing those issues and then carefully  assessed the evidence before the court. The  initial presentence report recommended a finding  that Mr. Roe had dealt 623.7 grams of cocaine  based on two separate factual components: (1)  Timothy Weger had been held responsible for 170  grams at his own sentencing, and had testified  that Mr. Roe had delivered that cocaine to him  over the course of six months; and (2) Weger also  testified that, in early April, he saw Mr. Roe  repackaging an additional pound of cocaine.2  The district court carefully considered the  evidence submitted with respect to both of these  components. Central to the district court's  analysis of both of these components was the  testimony of Timothy Weger. The court did not  accept that testimony blindly; it candidly noted  that there were some discrepancies and, in  evaluating those discrepancies, sought  corroboration in other circumstantial evidence.

2.

34
With respect to the 170 grams that Timothy Weger  said Mr. Roe had delivered over a six-month  period, the court noted that Weger had been  unable to remember exactly how much cocaine he  had in his possession at different times. The  court therefore considered other circumstantial  evidence with respect to that amount. It  considered Tina Weger's testimony that she "saw  Mr. Roe weighing out a quantity of cocaine  sufficient to fill a soft drink can in December."  R.101 at 57. In a later order, the court also  described Tina Weger as having seen Mr. Roe with  a "'Coke can' sized pile of cocaine." R.87 at 2.  Mr. Roe now argues that the district court  incorrectly recalled the trial testimony of Tina  Weger.


35
At trial, Tina Weger, when asked how much  cocaine she saw Mr. Roe weighing on a scale in  the basement of her home, made a hand gesture  that the court described as holding her thumbs  and first fingers together. Tina Weger then  responded, "It was more. It filled the whole  scale. I don't know how big it is. I don't know  how big, but it looked--or from here I mean it  looked like it was a nice-sized pile." R.96 at  24. The district court used terminology different  from Tina Weger's in describing the amount of  cocaine. That difference does not make its  finding of facts clearly erroneous. "We defer to  the sentencing judge's credibility determinations  because the presiding judge while listening to  the witnesses' testimony is in the best position  to observe, weigh, and evaluate a witness' verbal  as well as nonverbal behavior." United States v.  Pitz, 2 F.3d 723, 727 (7th Cir. 1993); see also  United States v. Magana, 118 F.3d 1173, 1205 (7th  Cir. 1997) (quoting Pitz), cert. denied, 522 U.S.  1139 (1998). The district court, after observing  Tina Weger's gesture and seeing the scale to  which she was referring, simply described in  graphic terms what it had seen.


36
In determining the extent of Mr. Roe's cocaine  dealing between November 1995 and April 1996, the  district court also found corroborative the  number of baggies for packaging cocaine that had  been found in Timothy Weger's trash. Weger had  testified that baggies were used to package  either 1/16 ounce or 1/8 ounce quantities. The  district court concluded that, even if all the  baggies were used to package the lesser amount,  three ounces (84 grams) would have been packaged  in a two-month time period. This rough  calculation, standing alone, would indeed be a  thin reed upon which to support a quantity  determination. Here, however, the court simply  employed this observation as a rough measuring  stick of the volume of cocaine involved in the  transactions in an effort to corroborate the  testimony of Weger. We cannot say that the  district court abused its discretion in using  this approach. Nor can we say that the district  court was wrong in its estimation that the 170  gram figure was "extremely conservative." R.101  at 56-57.

3.

37
The district court similarly engaged in a  careful process of evaluating Timothy Weger's  testimony concerning the amount of cocaine  possessed by Mr. Roe on April 4. It noted that  there were some inconsistencies. When asked at  trial how much cocaine Mr. Roe had on the night  of April 4, 1996, Weger said, "I don't know the  exact amount." R.72 at 37-38. However, at both  the trial and at the sentencing hearing, he  testified that, on April 4, he saw Mr. Roe with  two bags of cocaine that he would estimate  weighed four ounces, two bags that he estimated  weighed two ounces, seven or eight 1/8 ounce  bags, and one ounce left at the end of the night  for Weger's personal use. As Weger acknowledged,  there was an inconsistency between his trial  testimony and his testimony at the sentencing  hearing: At trial, he stated that on April 4 he  saw Mr. Roe with three "one-ouncers"; at the  sentencing hearing, he said he saw Mr. Roe with  only one. Including the ounce left for Weger, the  sum total of the cocaine involved would be a  total of either 17 ounces (485.7 grams) or 15  ounces (428.6 grams), depending on which version  is credited. The district court finally  characterized Mr. Weger's testimony as  "unwavering" that "he saw approximately 16 ounces  of cocaine bagged up that evening." R.101 at 58  (emphasis added).


38
As it had with respect to the quantity of  cocaine involved in the six month period, the  court took into account corroborating evidence in  determining the appropriate assessment of Weger's  testimony. It credited Timothy Weger's statements  that Mr. Roe had possessed 1 pound (457.1 grams)  in his basement on April 4. The court noted that  Weger was accustomed to seeing a variety of  smaller quantities of cocaine and, therefore,  would be able to estimate accurately the amount  of cocaine in a larger pile. Additionally, the  court pointed out that approximately 10 ounces of  inositol had been used in preparing the cocaine.  Mr. Roe does not dispute that the cocaine found  at the Weger residence was 45% pure. A cocaine  mixture of 45% purity containing 10 ounces of  inositol would weigh 18.18 ounces, or 519.48  grams.3 We think the district court was  entitled to conclude that it was "practically  indisputable" that there was "at least a pound of  cocaine involved." Id.


39
In determining the amount of cocaine involved in  the conspiracy, the district court credited  Timothy Weger's testimony after a thoughtful  examination of its contents and an evaluation of  the corroborating evidence. In proceeding in this  manner, it employed an acceptable methodology in  cases where there may be questions about the  credibility of an addict witness. See United  States v. Taylor, 72 F.3d 533, 544 (7th Cir.  1995). On appellate review, we shall disturb a  sentence only if we cannot find any evidence in  the record to support it and are left with a  definite and firm conviction that a mistake has  been made. See United States v. Joiner, 183 F.3d  635, 640 (7th Cir. 1999); United States v. Gabel,  85 F.3d 1217, 1221 (7th Cir. 1996). The evidence  in this case is sufficient to support the  district court's finding.

Conclusion

40
For the foregoing reasons, the judgment of the  district court is affirmed.

AFFIRMED


Notes:


1
 The Rule reads as follows:
Other crimes, wrongs, or acts. Evidence of other  crimes, wrongs, or acts is not admissible to  prove the character of a person in order to show  action in conformity therewith. It may, however,  be admissible for other purposes, such as proof  of motive, opportunity, intent, preparation,  plan, knowledge, identity, or absence of mistake  or accident, provided that upon request by the  accused, the prosecution in a criminal case shall  provide reasonable notice in advance of trial, or  during trial if the court excuses pretrial notice  on good cause shown, of the general nature of any  such evidence it intends to introduce at trial.
Fed. R. Evid. 404(b).


2
 A later revision to the presentencing report  recommended that Mr. Roe be held responsible for  only 396.9 grams, but that revision offered no  explanation for its new conclusion and the  district court did not rely upon it.


3
 Mr. Roe suggests that the cocaine originally  delivered to Weger's house may not have been 100%  pure; he points out that there is no testimony  that the cocaine delivered to the house was of  high purity. However, if we assume that the  cocaine to which the 10 ounces of inositol were  added was not 100% pure, this consideration only  raises the amount of cocaine mixture involved in  the conspiracy. As we noted above, assuming that  the inositol was added to 8.8 ounces of 100% pure  cocaine, adding 10 ounces of inositol to create a  mixture of 45% purity creates a cocaine mixture  of 18.18 ounces. Reducing the purity of the  original cocaine to which the inositol was added,  but maintaining the acknowledged constants of 10  ounces of inositol and a final mixture that is  45% pure, increases the weight of the original  cocaine mixture.


