X2
Tests Part II
Contingency X Test Of Independence Of Two Factors
Strickberger, M.W. 1968. Genetics. The MacMillan Co. New
York. pgs. 133-134.
The contingency test is a test for independence between two
factors. A significant X2
value indicates an interaction between factors. Interaction
means that the relative difference between two levels of one
factor are different due to the effect of the levels of a
second factor.
Example
The influence of environment on the segregation ratio of
a single gene is evaluated in two environments. The contingency
X2
test is used to evaluate whether the expected 3:1 segregation
ratio differs in different environments.
| Phenotype
of Offspring |
| Environment |
Dominant |
Recessive |
Totals |
| 1998 |
80 |
30 |
110 |
| 1997 |
90 |
25 |
115 |
| |
170 |
55 |
225 |
| Categories
of Observation |
| Environment |
Dominant |
Recessive |
Totals |
| 1998 |
a |
b |
a
+ b |
| 1997 |
c |
d |
c
+ d |
| |
a
+ c |
b
+ d |
a
+ b + c + d = N |
| N
= total number of observations |
