EPIDEMOLOGY

QUESTION

Complete the answers (show your working for all calculations, not just the answer) and forward them to the Education Office by the nominated due date.

We do not expect, or want you to write a mini essay for each response. Succinct answers which cover all the relevant points are desirable. However, make sure you show the formulae and all workings for any calculations undertaken. This may allow us to still allocate you marks even if your final answer was incorrect (in the unlikely event this would occur!).

Question 1

You are given the following figures regarding acute cerebro vascular accident (brain stroke) events in females in the city ‘A’ for 2001:

Total female population in the city ‘A’ for 2001:

For comparison, you are given the age-specific brain stroke incidence rates for the national female population (per 100,000):

You are interested in knowing whether the rate in females seen in the city ‘A’ is higher or lower than the national average.

a)         What sort of standardisation would you use? (1 mark)

b)         Carry out this standardisation, showing intermediate steps. (5 marks)

c)         What conclusion do you come to?  Is the brain stroke incidence for females in  city ‘A’  higher or lower then the national incidence? (2 marks)

Question 2

Seven hundred men aged 45 years were given a health check and 56 were found to have ‘type 2 diabetes’. Eight years later all 7,00 men (assuming none of them died during this period of time) attended a second check and another 112 men had developed ‘type 2 diabetes’.

i)                 What was the prevalence of ‘type 2 diabetes’ in the men a) at the age 45 and b) at age 53? (1+ 1 = 2 marks)

ii)               How many men were ‘at risk’ of developing ‘type 2 diabetes’ at the start of the 8-year period? ( 1 mark)

iii)             What was the incidence of ‘type 2 diabetes’ in these men? Is this a measure of cumulative incidence or incidence density? (1+ 1 = 2 marks)

Let’s assume that on average, each of the 112 men who developed ‘type 2 diabetes’ did so half-way through the 8 year follow-up period.

iv)              Calculate the total number of person-years at risk (of developing ‘type 2 diabetes’) during the 8 years. (1 marks)

v)               What was the incidence rate of ‘type 2 diabetes’ in these men? (1 marks)

SOLUTION

Table 1

Question 1

a>

Here I use indirect standardization.

b>

e=∑ e_i= total number of events in the study population (the observed number of events)=372

e_i= number of events in the i th age group of the study population=column 2 of table 1

e^(s)=∑ e_i^(s)=  total number of events in the standard population

e_i^(s)= number of events in the ith age group of the standard population

Ω_i^(s)= e_i^(s)/p_i^(s)= event rate in the ith age group for the standard population=column 5/100000

e^(s) /p^(s)= be the overall event rate in the standard population=1002.3/100000

P_i^(s)=total population of i th age group of standard population

E=expected number of events in the study population=

∑Ω_i^(s)*P_i=5.94+25.96+81.98+85.97+97.98+28=325.83

SER=standardized event ratio=(observed number of events/expected number of events)=e/E

=372/325.83=1.142

Standard error(SER)=√e/E=0.059

Indirect standardization rate(isr)=100000*SER* Ω^(s)=100000*1.142*0.010023=1144.62

Standard error(isr)=100000* Ω^(s)*standard error(SER)=100000*0.010023*0.059=59.13

c>

here indirect standardization rate is 1144.62 and standard error(isr)=59.13

and the observed stroke rate is calculated at column 4 of the table 1 is 1165.08. which is close to the indirect standardization rate. We cannot conclude at 5% level of significance that there be a valid diference between city A and national incidence.

Question 2.

(i)               a.

Prevalence is the number of existing cases of disease at a particular point of time.

So from this context I tell that 56 was the prevalence of ‘type 2 diabetes’ at the age 45.

b. 56+112=168 was the prevalence of ‘type 2 diabetes’ at the age 53.

(ii)             There are (700-168)=532 men were ‘at risk’ of developing ‘type 2 diabetes’ at the start of the 8-year period.

(iii)           Incidence is the number of new cases of disease within a specified period of time.

From this we conclude that another 112 men had developed ‘type 2 diabetes’ of the 8 year period is called the incident.

This is a measure of cumulative incidence.

(iv)            Total number of person- years at risk during the 8 years be

644+620+606+592+578+564+550+536=4690

(v)             14/644,14/620,14/606,14/592,14/578,14/564,14/550,14/536 was the incidence rate of ‘type 2 diabetes ’in these men.

JC33

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