Yesterday I did some backtesting of algorithmic trading systems to study 4-rule (n = 67) versus 2-rule (n = 65) strategies. I ran five two-sample t-tests assuming unequal variances (in Excel) to compare profit/loss (PNL), PNL/max drawdown, # trades, average trade, and profit factor.

Across the five tests, degrees of freedom varied from 76 to 125. I thought df was a defined number based on sample size, but obviously I'm wrong. Why would this vary?

Thanks,

Mark