Janaf,
The resistors I made for you and Andreas all come from the same spools of wire, i.e. 120R all from the same spool, 1K are all from the same spool, ect. for a total of four different spools of wire. Unlike other PWW resistor designs, all of my resistors exhibit the same characteristics without regard to the wire size, the 120R resistors have the same characteristics as the 1K, 12K or 70K, the majority (typically 60% or more) of the TCR will be within 0±1PPM/°C, therefor I can correctly claim that as a typical TCR with the remaining resistors within 0±3PPM/°C. I don't play word games with the specs and I use actual measured TCRs as the basis for the TCR spec, no box or butterfly hide and seek. The <1PPM/°C yield can be higher than 60% of course. Since PWW manufacturing is significantly different than film/foil, I cannot target the TCR characteristic directly, a given spool of wire may give me a tighter yield on TCR over another spool as such but that is a function of the wire supplier, the only control I have over them is my specification for the wire's TCR which is very specific, it is one of the reasons of many why I can turn out a significant low TCR yield without tweaking. I do not know of any other PWW resistor supplier that has as good a TCR yield as I do nor are they as linear in TCR, I repeat linear, that is inherent in the design and has been verified by 30 years of production and by customers. That linearity has been used to discover unknown non-linearity in circuits and measurements. The curve remains flat until near the outer temperature range when there is a slight change, this is mainly due to the alloy's characteristics.
Vishay does a lot of tweaking to their processes to produce lower TCRs, however, evidence would indicate that they do not have any consistent control over those processes that yield very low TCRs, particularly in the fact that few of their low TCR resistors are within the claimed 'typical' TCR spec. Whether Vishay actually does any 'cherry-picking' of parts from a given yield for those really low TCR parts, I cannot say for sure, I would definitely not expect Vishay to admit to it publicly or even privately for that matter. There is nothing wrong with selective picking of parts to meet a customer's specifications so I really don't know why Vishay denies doing it.
Yes, if a customer asks for a TCR yield tighter than my standard yield, then yes, they would have to be 'cherry-picked' from the batch. I can do things which can improve the yield of low TCR resistors in sets, for instance, if possible depending on the spread of the values, the 1K and 12K resistor set for example. One of my customers uses this to advantage, he has reported TCR tracking of better than <0.2PPM/°C over a 60°C range (all that he needed), copper adhesive tape was wrapped around the pair. I was not asked to guarantee a particular tracking TCR for this application, only what could I do to possibly improve tracking without high expense. As I've mentioned before, the tighter the specs, the lower the TCR specified, the higher the cost. While I do think that Vishay's oil filled are overpriced, that is the only way they are going to come close and I'd bet a dollar to a donut that those oil filled wonders are indeed cherry-picked. Are there resistors with TCRs close to zero in my yields, yes, how many? I really don't know, the distribution of TCR within a given range appears to be somewhat random in nature, there has never been any large scale measurements to mathematically indicate what kind of distribution is actually present. Given the fact that Vishay's foil resistors are made from essentially the same alloy as mine, they would also have a distribution of both + and - TCRs in their yields, what they are actually selling may be wholly different from that.