Simple random sampling and Stratified random sampling

Sampling frame

Nominal scale

In a study of the relationship between angular stomatitis and occupation, the occupation of each individual was recorded as : P (Professional), S (Skilled) or U (Unskilled). There were 88 people with angular stomatitis and 100 without the disease. The
distribution of 188 people by occupational  classification and angular stomatitis is as
follows :

Probability sampling

Double blind trial

Null Hypothesis