Show that the variance estimator XXXXXXXXXXfor the two sample tests is unbiased under the null hypothesis. Consider E 1 (t 0 ) given by (3.60). Show that   under the null hypothesis, that is, that...

Show that the variance estimator (3.55) for the two sample tests is unbiased under the null hypothesis.

Consider E₁(t₀) given by (3.60). Show that
  under the null hypothesis, that is, that E1(t0) can be interpreted as the number of events we would expect to observe in the first sample if the null hypothesis holds true.