Epidemiology module of practical skills for mbbs student

Practical exercise

Interpreting results: Confounding

Please refer to the following text and tables to carry out this exercise.

In a study of the epidemiology of tuberculosis in Denmark, Horwitz (Am Rev Respir Dis 1974;104:22-31) showed that tuberculosis was more common in men than in women and that it was much more common in those who were unmarried than in those who had been married. The results of a study determining the relation of marital status to tuberculosis notification in Canada from 1970-1972 is shown in table 1.

Table 1. Active tuberculosis (number of cases and average annual rate per 100 000) by gender and marital status, Canada 1970-1972

		Women					Men
Age group		Single	Married / Separated	Widowed / Divorced	TOTAL		Single	Married / Separated	Widowed / Divorced	TOTAL
All	No	1 122	3 140	703	4 965		2 113	4 641	640	7 394
	Rate	19,5	21,4	26,3	21,5		29,7	31,7	80,0	32,8

Grzybowski S. Evaluation of the Tuberculosis Problem and Control Measures in Canada. National Sanatorium Association, 1977

In table 2, the same data are displayed, in this case showing the distribution by age in addition to gender and marital status.

Table 2. Active tuberculosis by age, gender and marital status, Canada 1970-1972

		Women					Men
Age group		Single	Married / Separated	Widowed / Divorced	ALL		Single	Married / Separated	Widowed / Divorced	ALL
15-24	No	549	281	4	834		638	129		767
	Rate	13,3	15,5	13,8	14,0		12,6	13,5		12,7
25-34	No	229	732	44	983		372	565	13	950
	Rate	42,6	20,1	21,3	22,9		42.3	16,4	23,1	21,7
35-44	No	104	720	47	871		300	818	35	1 153
	Rate	39,3	21,8	29,7	23,4		79,0	24,1	44,1	29,9
45-54	No	74	607	77	758		300	1 038	85	1 423
	Rate	29,0	20,8	25,5	21,8		99.0	34,7	83,3	41,9
55-64	No	71	432	116	619		247	1 049	110	1 406
	Rate	28,3	23,2	22,3	23,5		102,2	47,9	84,1	54.9
65+	No	95	368	415	878		256	1 042	397	1 695
	Rate	30,8	32,5	28.7	30,4		102,9	61,9	96,3	72,3

1. 
   In table 1, is there a difference in notification rate according to marital status?
   
2. 
   
   
3. 
   Which group has the highest rate (specify one group by the 3 variables)?
   

_____________(age)__________(gender)__________(marital)

4. 
   Why do you think this is so?
   

5. 
   In table 2, are the conclusions you drew from table 1 valid? Yes [ ] No [ ]
   
6. 
   Which group in the table contributes the largest number of cases?
   

_____________(age)__________(gender)__________(marital)

7. 
   Which group has the highest risk for tuberculosis?
   

_____________(age)__________(gender)__________(marital)

8. 
   Why is there a difference?
   

9. 
   How would you modify the results displayed in table 1 to reflect reality?
   

One decade later, we further examined this association in the city of Vancouver. The results are presented in table 3.

10. 
    In table 3, are the conclusions you drew from table 2 valid? Yes [ ] No [ ]
    

11. 
    What is the cause of the difference in rates between ever married and never married men?
    

Table 3. Active tuberculosis(per 100 000 per year) in Vancouver 1980-1982 in adult Canadian-born men according to socioeconomic status and marital status.

	Ever married			Never married
Socioeconomic level	Number	Rate		Number	Rate
TOTAL	43	22		42	37
High	3	3		5	12
Middle	21	22		13	20
Low	19	299		24	326

Enarson DA, et al. Am J Epidemiol 1989;129:1268

Practical exercise

Interpreting results: Standardization of rates

The following table gives the situation concerning prevalence of significant skin reactions to tuberculin in residents of Honduras and of Canada, along with their populations:

Prevalence of significant tuberculin reactions in Honduras and Canada

		Canada				Honduras
Age	population	cases	rate (%)		population	cases	Rate (%)
TOTAL	27,000,000	8,100,000	30		3,510,000	1,072,275	31
0-14	1,890,000	0	0		1,120,000	43,971	4
15-24	5,130,000	0	0		910,000	115,379	13
25-34	6,210,000	0	0		700,000	295,400	42
35-44	4,320,000	540,000	13		390,000	262,681	67
45-54	3,780,000	1,890,000	50		210,000	183,204	87
55-64	3,510,000	3,510,000	94		110,000	101,640	92
65+	2,160,000	2,160,000	100		70,000	70,000	100

Calculate the rate of tuberculin skin reactivity in the two populations, standardized for age and enter into the table.
What is the explanation of the difference between the crude rate and the standardized rate?
What are the advantages of the standardized rates over the crude rates?
What are the disadvantages of presenting only the standardized rates?