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TiP and Beyond

This page explores uses of  the advanced time to most recent common ancestor (TMRCA) calculator that Family Tree DNA calls "TiP".

Accessing Tip

TiP is available only on the the private, secured portions of the FTDNA website, www.familytreedna.com. This means it is available only to FTDNA customers and project administrators.

TMRCA Comment

Some people are unhappy with the TMRCA concept; reality doesn't always match the predictions. We need to remember that these calculated probabilities are only maximum likelihood estimates (MLE) and further that their confidence limits are unknown. It's fair to say that any TMRCA is not a precise number but a window with an error of unknown size.

For example, a coin toss is expected to have a  50% probability coming up heads. But in any series of tosses, there may be more heads or tails. Only as one increases the tosses to a  large number  does the percentage of heads (and tails) approach 50%.

What is TiP?

Like other such calculators, TiP compares two sets (a pair) of Y-DNA STR results to compute a maximum likelihood estimate (MLE) for a cumulative probability that the most recent common ancestor (MRCA) lived within a specified number of generations.

Advantages

Disadvantages

Mechanics

TiP utilizes a Gamma distribution model for its probability estimates. (For more on gammas, see this page.) The two parameters (α & β) for the distribution are related to the number and type (e.g., marker volatility, palindromic) of haplotype differences. For more mismatches or mismatches on more stable markers, the parameters used increase to reduce probability at a given number of generations. .

Other TMRCA calculators also use Gamma distributions but their parameter handling is less sophisticated.

 

Advanced TiP

TiP can be used in more than one way. For these uses, we recommend:

  1. Use the "per single generation" option to produce TiP reports for every generation from 1 to 24. (Do not use the "not related within" option; this adjustment can be made later.)
  2. Copy and paste the output to a spreadsheet file (like Excel's)
  3. Transcribe the text output into numbers-only columns for generations and cumulative probability.

These steps enable calculations for further use of the data.

Assessing match quality

Your matches may be listed in order of "genetic distance" from you; those at the top are not necessarily your closest relations. For any given number of generations, your closest relations will be those with the highest cumulative probability as given by TiP.

Focusing research

More preparatory steps are needed to focus research to find the identity of the MRCA

  1. Calculate per-generation probabilities; use another column for this: Subtract the cumulative probability of each generation from its more recent neighbor. For example subtract the probability for generation 5 from 6 to get a per-generation estimate for 6.

However, these numbers are not particularly reliable or precise. So another step is needed.

  1. Sum individual per-generations probabilities into adjacent groups. For example, sum 3,4 & 5.

One group of generations will stand out as most probable for where in the family tree the MRCA is to be found. A group of three generations for example, yields a time window of about 100 years.