Epidemiology Report

This report analyzes various epidemiological data, including incidence rates of diseases in different regions, comparisons of health risks, and the impact of demographic factors on disease prevalence. It includes calculations and interpretations of data related to gastroenteritis, syphilis, and HIV/AIDS, providing insights into public health implications and the importance of understanding epidemiological trends.

Running head: EPIDEMIOLOGY
EPIDEMIOLOGY
[Author Name(s), First M. Last, Omit Titles and Degrees]
[Institutional Affiliation(s)]

2
EPIDEMIOLOGY
Q.1 “This table presents the incidence rate IR (cumulative) and population size for two
regions”
“Which of the following statements are True (if Any) – explain your answer by calculation if
needed”
a. There were the same number of cases in both regions [50 words]
The incidence rate in Region A is calculated as (10/100) ×10000=1000 people while
the incidence rate in Region B would be determined as (5/100) ×5000=250. It is thus
false to say that the incidence rate in the two regions is the same. From the
calculations, the incidence rate in Region A is higher at 1000 comparatively to Region
B which stands at 250 (Abramson, 2012).
b. There were twice as many cases in region A then region B [50 words]
The statement is false. From the calculations done in (a) above it has been found that
the incidence rate at Region A is 1000 while that at Region B stands at 250. The
prevalence in Region B is thus found to be a quarter the prevalence in Region A. In
other words to arrive at the prevalence in Region B, the incidence rate in Region A is
divided by 4 and vice versa to get the incidence rate in Region A (Wilkinson, 2013).
c. The risk of incurring the disease during the year was about the same for individual in
region A as for those in region B [50 words
Since the annual incidence rate in Region B was half that in Region A, the risk for
individuals was much higher than, as high as twice, in region A (Jamison, 2016). the
incidence in Region A is given as 10 persons for every 100 while at Region B is 5
Region A Region B
Population size 10,000 5000
IR of gastroenteritis 10 per 100 5 per 100

3
EPIDEMIOLOGY
persons for every 100 at different demographic values. This statement is therefore
false.
d. The risk of incurring the disease during the year was twice as high for individuals in
region A as for those in region B. [50 words]
Since the annual incidence rate in Region B was half that in Region A, the risk for
individuals was much higher than, as high as twice, in region A. the incidence in
Region A is given as 10 persons for every 100 while at Region B is 5 persons for
every 100 at different demographic values (Jamison, 2016). This statement is
therefore true. The risk of incurring the disease was twice higher for individuals in
Region A as in region B.
Q2 – “What will be the incidence rate of region A and Region B combined? [150 words]
Regions A and B combined gives 1000+250=1250 cases. This means there is a total of 1250
individuals who are at a risk of the disease infection in the two regions during the year
cumulatively (Vogt, 2012). The total population of the two regions is 10000+5000=15000.
Using these statistics, the overall incidence rate can be calculated as
(1250/15000)*100=8.333%. This is loosely translated as 8.33 persons per 100. The overall
incidence rate is determined as the weighted mean of the two different rates and using the
sizes of the population as weights. The size of a subpopulation is a factor in determining the
effect of a subpopulation in the total population (Abramson, 2012).
Q3. “In an army base, where there is a complete change of personnel every 3 months and the
total base is always 1000 (i.e., every 3 months 1000 new soldiers arrive and 1000 leave). It
was found that 2000 soldiers incurred syphilis every year. What is the yearly cumulative
incidence of syphilis in this base? “ [100 or 150 words]

4
EPIDEMIOLOGY
To calculate the cumulative incidence rate of syphilis in this base, the incidence rate within
the 3-months stay at the base is determined. In a year there will be 4000 soldiers in the case
who are followed up after every three months. Out of this number, 2000 soldiers are found to
contract syphilis (Abramson, 2012). The cumulative incidence rate in the base after every
three months is thus calculated as (2000/4000)= 50 cases of risks for every 100 soldiers.in
other words, the incident rate is 50% which indicates the risk of an individual to developing
syphilis during three months of his stay at the base.
Q4. “Now assume that in each three-monthly batch of 1000 soldier there were 250 soldiers
who contracted syphilis after exactly one month (let’s say on the payday) and another 250
who did so after precisely two months”.
a. Calculate the sum total of the soldiers’ period of exposure to risk (person-time) for
use as denominator [50 words]
In a single cohort of 1000 soldiers, 250 are at risk for 1 month, 250 at a risk for 2 months
and the remaining 500 at a risk for the full 3 months. Therefore each batch is exposed to
risk for (500*3) + (250*2) + (250*1) =2250 soldiers-months. This is used as the
denominator (Vogt, 2012). The numerator which is the number of cases is 500 and the
rate is thus 500/2250=0.222 per soldier=months.
b. Calculate the person-time incidence rate [50 words]
The person-time incidence can be calculated for the 3-month period. From the preceding
calculations, the rate can be said to be 0.67 (67%) for every 3 person-months. Using this
rate as a rough estimation of the risk of a soldier of incurring syphilis during a three
months period, calculation of the corresponding cumulative incidence rate can be done.
The cumulative corresponding incidence rate gives a more direct measure of the risk of

5
EPIDEMIOLOGY
contraction (Woodward, 2013). The cumulative incidence rate for this data stands at 0.50
which is 50%.
Q5. “The following table depicts the age distribution and the number of HIV/AIDs cases that
exist in each age group”
Intravenous drug (IVD) users Homosexual (men to men sex)
Age group
(years)
Population size # HIV/AIDs
cases
Population size # HIV/AIDs
cases
18-30 55 4 112 30
31-55 8 1 141 32
56+ 2 0 95 15
a. “Compare the crude prevalence of HIV/AIDS between the two groups? (Show the
process of calculation, not just one final number)" [50 words]
The crude prevalence is determined by the total number of people at risk divided by the total
population at a given time. The crude prevalence for IVD users is therefore determined from
the expression (4+1+0)/ (55+8+2) =5/65=0.0769 while the crude prevalence in the
Homosexual population is determined by (30+32+15)/112+141+95=77/348=0.2213. It is thus
evident from the calculations that the crude prevalence is higher in the homosexual
population than in IVD users’ population (Wilkinson, 2013).
b. “Compare the age-specific prevalence between the groups, is there any pattern?” [50
words]
From the above data, the age-specific rates are not identical hence does not exhibit any
pattern (Jamison, 2016). However, it can be noted from the data that the prevalence rate is

6
EPIDEMIOLOGY
quite higher in older people in both populations. Homosexual population is more heavily
weighted with older people and the age tends to be associated with the risk of HIV/AIDS
contraction. In this light, more people in the homosexual population have a higher risk of
contracting the disease.
c. “Would it be correct to compare the crude prevalence between the two group?
Explain why yes or why not (no more than 60 words)”
It would not be correct to use crude prevalence in making a comparison of prevalence
between the two groups (Abramson, 2012). Crude prevalence rate gives unfair comparison of
the cases in the two groups because of unequal distribution of the ages in the data. Unequal
distribution of the ages may result into non-comprehensive analysis.
d. “Compute the direct standardised prevalence of HIV/AIDs for intravenous drug users
using the age distribution of homosexuals. Express your results in percent”.
Dividing the expected cases of HIV/AIDS back by 100
Intravenous drug
(IVD) users
Homosexual (men to
men sex)
Age
group
(years)
Population
size
#
HIV/AID
s cases
Population
size
#
HIV/AID
s cases
18-30 55 4 0.04 0.04*50=2 112 30 0.3 0.3*50=15
31-55 8 1 0.01 0.01*40=0.4 141 32 0.32 .32*40=12.8
56+ 2 0 0 0*5=0 95 15 0.15 .15*5=.75
Total
expected
cases
2.4 28.55
Adjuste
d age
rates
2.4/95=0.0253 28.55/95=0.3
Adjusted standard age ratio=0.0253/0.3=11.89%
Using a standard population table below

7
EPIDEMIOLOGY
18-30 50
31-55 40
56+ 5
Total 95
e. Summarise the differences between the crude prevalence and the age-standardized
rate. (no more than 100 words)
The difference between the crude prevalence rate and the age-standardized rate for the two
populations could be attributed to the significant difference in the age structures of the two
populations. The homosexual population has a much older population that the IVD users'
population (Woodward, 2013). This leads to the gap observed in the two prevalence rates of
the different populations.
Q6. Read the following abstract and answer the following questions
Background: Although gait speed slows with age, the rate of slowing varies greatly. To date,
little is known about the trajectories of gait speed, their correlates, and their risk for mortality
in older adults.
Methods: Gait speed during a 20-m walk was measured for a period of 8 years in initially
well-functioning men and women aged 70–79 years participating in the Health, Aging, and
Body Composition Study. We described the trajectories of gait speed and examined their
correlates using a group-based mixture model. Also, risk associated with different gait speed
trajectories on all-cause mortality was estimated using a Cox-proportional hazard model.
Results: Of 2,364 participants (mean age, 73.5±2.9 years; 52% women), we identified three
gait speed trajectories: slow (n = 637), moderate (n = 1,209), and fast decline (n = 518).

8
EPIDEMIOLOGY
Those with fast decline slowed 0.030 m/s per year or 2.4% per year from baseline to the last
follow-up visit. Women, blacks, and participants who were obese had limited knee extensor
strength and had low physical activity were more likely to have fast decline than their
counterparts. Participants with fast decline in gait speed had a 90% greater risk of mortality
than those with slow decline.
Conclusion: Despite being well-functioning at baseline, a quarter of older adults experienced
fast decline in gait speed, which was associated with an increased risk of mortality.
a) What is the study design - what led your decision of the study design from this text
[2 marks]
The study design to be used in this case is cross-sectional studies. From the above
extract, a study is done between two groups of persons, one exposed to disease
prevalence and another which is not exposed to the prevalence (Congress, 2015). The
study design is as well effective for significant persist conditions as expressed in the
results noted in the extract.

9
EPIDEMIOLOGY
References
Abramson, J. H. (2012). Making Sense of Data: A Self-Instruction Manual on the
Interpretation of Epidemiological Data. London: Oxford University Press.
Congress, U. S. (2015). Congressional Record: Proceedings and Debates of the ... Congress.
Washington: U.S. Government Printing Office.
Jamison, D. T. (2016). Disease Control Priorities in Developing Countries. London: World
Bank Publications.
Vogt, W. P. (2012). When to Use What Research Design. Manchester: Guilford Press.
Wilkinson, R. G. (2013). Social Determinants of Health: The Solid Facts. New York: World
Health Organization.
Woodward, M. (2013). Epidemiology: Study Design and Data Analysis, Third Edition. New
York: CRC Press.