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The biggest variable that I've seen is the cM conversion table being used. Everybody uses a different one, it seems. That means you may be comparing apples to oranges, to some extent. Below is a sampling I just took of parent/child values for 22 chromosomes:
FTDNA 3384.3
23andMe 3546
GEDmatch 3587.1
You may see minor variations in the values, but these represent a reasonable comparison between the different conversion tables.
After the parent/child values, the total cM will reduce approximately half by each generation. That's a logarithmic relationship. As you get further away from the first generation, you will see more and more variability (as demonstrated in John Walden's graph), due to random inheritance differences, noise, and even multiple inheritance paths.
FTDNA 3384.3
23andMe 3546
GEDmatch 3587.1
You may see minor variations in the values, but these represent a reasonable comparison between the different conversion tables.
After the parent/child values, the total cM will reduce approximately half by each generation. That's a logarithmic relationship. As you get further away from the first generation, you will see more and more variability (as demonstrated in John Walden's graph), due to random inheritance differences, noise, and even multiple inheritance paths.
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