When looking at a population of hundreds of lineages, generations per century may be a floating concept; some lineages will coalesce around 3 while others will coalesce around 4 and even 5. Using 2 pedigrees with expectation to extrapolate the information as such into a population at large isn't exactly advised. I hope this discussion isn't intended to ramp up TMRCA or STR wars and I'll not engage in that; however in that regard, when using statistical considerations for estimating TMRCA for a population at large in which there is varying generation lengths, the averages must be used for statistical purposes. Each method will incorporate into the method of calculation a generation length constant; in that regard, however it makes little difference whether or not the generation length is 3 or 4 per century to calculate the TMRCA. Most generation length averages will be tied to a mutation rate constant for calculating purposes and all are subject to a standard deviation as well as all statistical considerations are. If using a pedigrees, counting generations and attempting to achieve by such how old that pedigree is within the genealogical period is one thing. If using a statistical method to calculate ages using the haplotypes or a combination of SNPs and haplotypes, that is another thing all together, and one uses the average generation length the method calls for. If calculating a lineage or groups of lineages, the genealogies are used as a cross check and calibration of the method is advised. This calibration may then be undertaken (if adjustment is necessary) either by the method of mutation count, adjusting the mutation rate constant or both. Rarely have I ever had to adjust a mutation rate constant as the mutation count method adjustment is generally sufficient to accomplish the task. I also might add that calculating a lineage and calculating a population have different considerations and are approached to meet those considerations. Susan Hedeen On 5/18/2014 10:37 AM, Bernard Morgan wrote: >> So, we have two independent pedigrees which indicate that the generation >> time M222 people are interested in was a lot closer to 3 generations per >> century than to 4 generations per century. Incidentally, I could show that >> 3 generations per century held up over different parts of my pedigree - in >> Iceland , in the USA and Canada and in ancient Ireland. >