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  #1  
Old 16th January 2017, 06:47 PM
Gammon Gammon is offline
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Join Date: May 2015
Posts: 2
Did not share a common ancestor in the past X generations - TiP Report

Hi, Can you please help me understand the results of the Y-DNA TiP report. I have read all of the FAQs in the Learning Centre but the TiP report results contradict the information provided. For instance, if I use the default of “did not share a common ancestor within the last 1 generation” the results are:

COMPARISON CHART
Generations Percentage
1 12.94%
2 29.51%
3 45.42%
4 59.01%

This shows a 12.94% chance within the last 1 generation but shouldn't it be zero?

https://www.familytreedna.com/learn/...-ftdnatip-use/

Similarly, if I follow the example from the FAQ (link above) and exclude a common ancestor within the past three generations, it logically states that the common ancestor must be at least four generations back. But my results are as follows:

In comparing Y-DNA 37 marker results, the probability that xxxx and Dr. Roger Barry Gammon shared a common ancestor within the last...

COMPARISON CHART
Generations Percentage
3 22.57%
4 41.85%
5 57.27%
6 69.12%

xxxx and Dr. Roger Barry Gammon did not share a common ancestor in the last 3 generation(s).


I have excluded the chance of a common ancestor within 3 generations but it is showing a 22.57% chance.

I’m trying to understand whether there is an error in the TiP calculator or in the description of how it is used.

Regards,
Philip
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  #2  
Old 2nd March 2017, 01:22 PM
mouseinhouse mouseinhouse is offline
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Join Date: May 2016
Posts: 11
Yes, the results don't make logical sense given the descriptions of the tool. The only sensible explanation I've heard is that the tool generations probability curves and doesn't cut them off at the place you'd expect. My own approach is to discard that first data point, and take the whole TiP analysis with a couple grains of salt for close matches.

All the data entry field for “did not share a common ancestor within the last N generations" does is shift the curve outward toward more generations and then cut off the data at the number of generations you've specified. Also, note the degree of precision shown in the results, two decimal places, is totally bogus and misleading. This tool provides only rough estimates, based on a necessarily limited model for STR changes.

You may find some plotted TiP curves useful for getting a feeling for how the analysis works. Putting the results table in MS-Excel for example and charting them while changing various parameters can be helpful for getting a better feel for the value and limits of TiP analysis.
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Old 2nd July 2017, 02:31 PM
MontyFrost MontyFrost is offline
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Join Date: Jun 2017
Posts: 1
It is a shame FTDNA allows this.

Thank you to the customer mouseinhouse who tried to add some clarity to the gross inaccuracies in the TiP reports that show a non-zero probability of relations when the paper trail excludes them. It is beyond me how FTDNA could stand behind the TiP analysis when it is clearly badly in error. I wish FTDNA could provide a TiP analysis that is logically consistent.

On a separate note, I have had Y-DNA STR analysis done on four distant cousins, including myself, who the paper trail indicates are likely to have ancestors who are brothers, i.e. all four may have a common paternal ancestor. Is there any further analysis that can be done on the detailed STR results of these four people to show the likelihood that they have descended from the same paternal ancestor? I am thinking about the use of the STR change information, none of which are the same across the 4 people.
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