Behavioural Surveillance Surveys - The Wisdom of Whores

Step 4 : Add together the result of that calculation for every cell of the table. The result is the
Chi-square value. The total of all the cells (excluding, of course, the row and column totals) is the
sum of all the column OR row totals, found in the bottom right hand corner of the table. So in this case :
χ 2 = 49.37.
Step 5 : In order to determine whether this χ 2 value is statistically significant, it is necessary
to calculate the degrees of freedom.
DF = (number of rows - 1) * (number of columns - 1)
DF = (4-1) * (4-1) = 9.
Step 5 : Go to the Table Chi-Square and look up the significance level for a χ 2 value of 49.37,
with nine degrees of freedom. For nine degrees of freedom, the largest value given is 27.88,
for a significance level (or p-value) of 0.001. Since the value we have calculated is larger than this,
it means that it is significant at even greater levels of significance than that given. A p-value of
0.001 corresponds to a level of certainty of 99.9 percent. In other words, by using this test,
we have shown that we are more than 99.9 percent sure that the association between age
and non-regular sex partners could not have occurred by chance. This means that program
planners in Côte d’Ivoire can with confidence assume that younger truck drivers are more likely to
have multiple non-regular sex partners than older truck drivers, and they can target their prevention
activities accordingly.
There are some limitations on the validity of this test. Chi-square distribution tests cannot
reliably be used where more than a fifth of the values in the table of expected values are expected
to be smaller than five, or when any expected value is less than one.
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A PPEN DI X 5 B EHAV I OR A L SURV EI L L A NC E S U R V EY S