Question

1. Case control study by Smeeth et al (Lancet 2004)
1294 cases of pervasive developmental disorder (PDD) and 4469 controls were included.
1010 cases (78·1%) had MMR vaccination recorded before diagnosis, compared with
3671 controls (82·1%) before the age at which their matched case was diagnosedOver 25 studies have been conducted which have failed to show an association
between MMR vaccine and autism. Below are some data from two of these studies
(one cohort and one case-control). Calculate the measure of association for each
study after completing the 2x2 table and interpret the measure of association.
2.. Cohort study by Madsen et al (NEJM 2002)
Of the 537,303 children in the cohort, 440,655 (82.0 percent) had received the MMR
vaccine. We identified 316 children with a diagnosis of autistic disorder; 263 of the cases
had been vaccinated with MMR (83%); 53 children with autism had not been vaccinated

Answer 1

1)

(A.) A 2X2 table is:

	PDD+(Case)	PDD-(Non- Case)	Total
MMR+(Exposed)	1010 (A)	3671 (B)	4681 (A+B)
MMR-(Unexposed)	284 (C)	798 (D)	1082 (C+D)
Total	1294 (A+C)	4469 (B+D)	5763{(A+C)+(B+D)}

B. The measure of association is

=[(AD)/(BC)]

=((1010x798)/(3671x284)

=0.773

Measures of association provide a means of summarizing the size of the association between two variables. Most measures of association are scaled so that they reach a maximum numerical value of 1 when the two variables have a perfect relationship with each other. They are also scaled so that they have a value of 0 when there is no relationship between two variables.