Here is the common scenario: a co-conspirator sells some user quantity of methamphetamine to an informant, then is arrested. The lab tests the meth as 99% pure, because everything is these days. Then, the co-conspirator snitches on the others, because everyone does these days. And in the debriefing, he is questioned about amounts: yes, one to two kilos are brought in every week and distributed through the ranks to sell on the street. This went on for about four to six months. He thinks. The PSR exercises conservative restraint and only counts the lower estimates: one kilo a week for 12 weeks equals 12 kilos, in addition to the few ounces actually seized.
This 12 kilos is ghost dope -- somebody said it existed at one time, and they saw it or some evidence of it, but they can't show it to the police or, more importantly, to the laboratory or the jury. Their word on the total amount may hold (not that this should all be attributed to your individual client, even if he is a co-conspirator, see here).
But purity is the game in meth guideline calculations. With 12 kilos, the difference is a base offense level of 34 for a mixture or 38 for actual or ice. Here is the fight: the government argues that the purity of the seized dope, or of average meth on the streets, must also be the purity of the ghost dope. But this argument is "a bridge too far", at least without some corroboration or connection to the defendant, according to United States v. Lemus out of the Ninth Circuit (March 2, 2016). In Lemus, the court rejected a purity approximation that was unconnected with the defendant. Although it found that there was, indeed, sufficient evidence to justify a conviction for distribution of methamphetamine conviction, there was not evidence to justify the quantity element of over 50 grams. The amount depended on the purity of ghost dope, and the only evidence was from the agent who testified that almost all meth seized in the area was more than 90% pure. "It would be a bridge too far to allow a jury to extrapolate from comparison drugs that were not from activity related to the defendant or a conspiracy in which the defendant is involved." The quantity element of the verdict was vacated.
Moreover, on remand, double jeopardy precludes retrial on the higher quantity and lower court was instructed to recalculate the using the lowest quantity category in section 2D1.1. Ghosts did not carry the government's case.
Of course, meth wasn't the first ghost in a courtroom: