Somali blood groups


H0 :   π  =  0.574
HA :   π  ≠  0.574

Use this diagram to illustrate the doubling of the tail area to find the p-value for a 2-tailed test.

Drag the slider to x = 26, the observed number with blood group O. The bar chart highlights the smaller of the two tail areas and this tail probability is doubled to give the p-value (0.2169).

Since there is over 21% probability that the sample proportion is this far from the population proportion, there is no evidence that this sub-population is genetically different.


In a study of sab bondsmen, a population sub-group in Northern Somalia, blood tests were conducted on a sample of 54, in order to investigate whether they differed genetically from the main population of 'free-born noble Somali'.

It is known that a proportion 0.574 of free-born noble Somali have blood group O. (Actually 574 had blood group O in a sample of 1000, but this sample size was large enough to provide a reasonably accurate estimate.) Is there any evidence that the sample proportion with blood group O in the sab bondsmen, 26 out of 54, does not come from a population with π = 0.574?