tstatistics 
Prob. is the twosided probability of the absolute value of the reported t with the reported d.f., so that a statistically significant finding for a single twosided ttest is Prob.<.05, and a highly significant finding is Prob.<.01. Please interpret this conservatively, i.e., "barely significant is probably not significant". If you wish to make a Bonferroni multiplecomparison correction, compare this Prob. with your chosen significance level, e.g., p<.05, divided by the number of DIF tests in this Table. This is approximate because of dependencies between the statistics underlying the computation.
Many statistical tests are reported as Student's t statistics. This table shows the significancelevel values for different degrees of freedom (d.f.). Often the reported tstatistics have effectively infinite degrees of freedom and so approximate a unit normal distribution. tstatistics with infinite degrees of freedom are also called zstatistics, paralleling the use of "z" in zscores.
Table of the twosided t distribution 




A calculator for the probability of any t value and d.f. is at http://www.danielsoper.com/statcalc3/calc.aspx?id=8
Welch's refinement of Student's ttest for possibly unequal variances:
For sample 1,
M1 = mean of the sample
SS1 = sum of squares of observations from the individual sample means
N1 = sample size (or number of observations)
SS1 / (N1  1) = sample variance around the mean (or the measure)
SS1 / ((N1  1)(N1)) = standard error variance = EV1 = SE1²
SE1 = Sqrt(EV1) = standard error of the mean (or the measure)
Similarly for sample 2, then
t = (M1  M2) / sqrt (EV1 + EV2) = (M1  M2) / sqrt (SE1² + SE2²)
with WelchSatterthwaite d.f. = (EV1 + EV2)² / (EV1²/ (N11) + EV2² /(N21))
which is the same as d.f = (SE1² + SE2²)² / (SE14 / (N11) + SE24 / (N21))
A calculator for this is at http://www.graphpad.com/quickcalcs/ttest1.cfm?Format=SEM
Satterthwaite, F. E. (1946), "An Approximate Distribution of Estimates of Variance Components.", Biometrics Bulletin 2: 110114
Welch, B. L. (1947), "The generalization of "Student's" problem when several different population variances are involved.", Biometrika 34: 2835
Example: Gender subtotals for Example0.txt Table 28:
M1 = 1.62, M2 = .76, SE1 = .38, SE2 = .16, N1 = 18, N2 = 57
Welch: t = 2.08, d.f. = 23, p = .049
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