Question

In January, 1st, 1990, investigator A took a simple random sample of 1,050 adults from an urban community of 100,000. After examining the entire study population of 1,050, she had detected 50 cases of diabetes, a chronic disease for which there is no recovery or cure. In January, 1st , 1995 (5 years later), she re-examined all of the participants from her original study population. Thirty new cases of diabetes were identified. Ten subjects died (showed in the Table 2) during the 5 years. One person (No. 7) died from diabetes. No one was lost to follow up.

1) Please calculate the prevalence rate of disease Q in 1990 and in 1995 (2 points).  

2) Please calculate the cumulative incidence rate of diseases Q from 1990 to 1995 (1 point);

3) Please calculate the incidence density for disease Q (2 points).

Table 2. The exact dates for all deaths between January, 1st 1990-January, 1st, 1995.

NO	Death date	Causes of Death
1	1991-01-8	Other reasons
2	1992-09-01	Other reasons
3	1992-12-01	Other reasons
4	1993-02-03	Other reasons
5	1993-12-11	Other reasons
6	1993-12-31	Other reasons
7	1994-05-01	Diabetes
8	1994-05-25	Other reasons
9	1994-06-30	Other reasons
10	1994-08-13	Other reasons

Answer 1

Total selected People = 1050

In 1990 Diabetics patient = n1 = 50

In 1995 Dibetics patient = n2 = 50+30 = 80

Prevalence Rate = [(no. of dibetics patient) / (Total of sample)] * 100

Cumulative Incidence Rate = [(no. of new dibetics cases) / (Total of sample - Initial knowen patient)] * 100

Incidence Density = (no.of new cases) / (Total of sample per time unit)

For Year 1990:

Prevalance Rate = [50/1050] * 100

= 4.7619 %

For Year 1995:

Prevalance Rate = [80/1050] * 100

= 7.6190 %

I.e Prevalance rate is increased (2.85%) in the year 1995

2) Cumulative Incidence Rate = [ 30/1000] * 100

= 3 %

Incidence Rate is 3% in last five year from 1990 to 1995

3) Incidence Density = 30/5 -Year

= 6 per Year

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