How much deviation triggers rejection of the null hypothesis?
A probability of 0.05 (5%) that the observed deviation was a random chance event corresponds to a X2 value of 3.84. In the example presented above, a deviation (O-E) of 27 corresponds to a X2 value of 3.65, still within the limit, whereas a deviation of 28 corresponds to 3.92, just outside the cutoff point for rejection of the null hypothesis. This means that for the current sample size, a phenotypic ratio of 759:305 (2.49:1) or 915:249 (3.67:1) would be rejected as not matching a 3:1 expectation, whereas anything between those values would be accepted as “close enough” to satisfy the chi-square test at P = 0.05. “Highly significant” rejection of the null hypothesis: Boxed example 12.8 applies chi-square analysis to the example of linkage described in boxed example 12.4. The chi-square value that is obtained indicates that the probability that the results came from chance deviation is far less than 0.01. This supports the conclusion that the rejection of the null hypothesis is highly s
