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Advanced statistical methods in epidemiology

Assessment Exercise

For the assessment exercise, you are asked to analyse data from a cohort study of childhood malaria, carried out in a rural population in Northern Ghana.

Infants residing in a random sample of households in the study area were enrolled in the cohort when they attended a health facility for their first DPT vaccination. Follow-up was through passive case detection at local health facilities. An episode of clinical malaria was diagnosed if a child presented to a facility with fever and if malaria parasites were detected on microscopy. Data on potential risk factors for clinical malaria were obtained from a household survey carried out in the same study area.

A total of 875 infants were enrolled between September 2000 and June 2002, and follow-up continued until June 2004.

You are asked to use the data from this cohort study to address the following questions:

  1. What was the incidence rate of clinical malaria in this cohort?
  2. How did this rate vary over time?
  3. What were the risk factors for clinical malaria in this cohort?
  4. Was there any evidence that the incidence of clinical malaria was either increased or decreased if children had experienced a previous episode?

In addressing these questions you may use either or both of the datasets provided.

The data

Two datasets are provided. The first is restricted to the first event of clinical malaria experienced by each child during the follow-up period. The second gives data on all events.

Important note: Please make sure you download these two new datasets from the U: drive on or after Friday 22 April as these replace earlier datasets that are now obsolete.

The Stata dataset ghana.first.2016.dta contains 875 observations and 21 variables as shown below. In this dataset, observation ends at the time (devent) of the first episode of malaria (malepi = 1) or exit from the study (malepi = 0).

Variable name

Description

Coding

id

Unique identification number

hhsize

Number of household members

nroom

Number of rooms in household

ethnicity

Ethnicity of household head

1 = Kassem

2 = Nankam

3 = Buli

4 = Other

religion

Religion of household head

1 = Traditional

2 = Catholic

3 = Muslim

nhis

Health insurance status of household members

1 = Yes, all

2 = Yes, household head

3 = Yes, others

4 = Yes, household head and others

5 = None

water

Source of drinking water of household

1 = Bought

2 = Piped water

3 = Borehole/well

4 = Surface water

electric

Source of light for household from electric grid

1 = Yes

2 = No

tele

Household possession of television

1 = Yes

2 = No

radio

Household possession of radio

1 = Yes

2 = No

netsum

Number of bednets possessed by household

netuse

Study child uses bednet

1 = Yes

2 = No

itnuse

Study child uses insecticide treated bednet

1 = Yes

2 = No

urban

Urban residence

1 = Yes

2 = No

dob

Date of birth

sex

Sex of child

1 = Male

2 = Female

doe

Date of enrolment

vistat

Vital status at exit from study

1 = Alive

2 = Died/Migrated

doexit

Date of exit from study

malepi

Episode of clinical malaria

0 = Exit from study

1 = Episode of malaria

devent

Date of first episode or exit from study

The Stata dataset ghana.all.2016.dta contains 2522 observations and 23 variables as follows:

Variable name

Description

Coding

id

Unique identification number

hhsize

Number of household members

nroom

Number of rooms in household

ethnicity

Ethnicity of household head

As above

religion

Religion of household head

As above

nhis

Health insurance status of household members

As above

water

Source of drinking water of household

As above

electric

Source of light for household from electric grid

As above

tele

Household possession of television

As above

radio

Household possession of radio

As above

netsum

Number of bednets possessed by household

netuse

Study child uses bednet

As above

itnuse

Study child uses insecticide treated bednet

As above

urban

Urban residence

As above

dob

Date of birth

sex

Sex of child

As above

doe

Date of enrolment

vistat

Vital status at exit from study

As above

doexit

Date of exit from study

start

Start date of time period

end

End date of time period

malepi

Episode of clinical malaria

0 = Exit from study

1 = Episode of malaria

prevmal

Number of previous episodes of clinical malaria since enrolment

In the above dataset, there are multiple records for children who experienced an episode of clinical malaria. Each record corresponds to a period at risk, and ends either with a malaria episode (malepi = 1) or exit from the study (malepi = 0).

There were 875 children and a total of 1648 episodes of clinical malaria: x 200 children had no episodes x 227 children had 1 episode x 188 children had 2 episodes x 123 children had 3 episodes x 76 children had 4 episodes x 31 children had 5 episodes x 30 children had more than 5 episodes

Background notes

Malaria is caused by a parasite transmitted by the bite of an infected anopheline mosquito.

Worldwide it causes around 600,000 deaths each year, mostly among young children in subSaharan Africa. After repeated infections, children gradually develop partial immunity to the parasite. Not all infections give rise to severe symptoms, and not all symptomatic cases are seen or treated at health facilities.

The project report

Your report on this analysis should be printed on A4 paper (single-spaced), and must be no more than 5 pages including any tables or figures. Small fonts and small margins must not be used. The report should include:

  1. A brief discussion of the strategy you used in analysing the data (maximum 1 page). This should be more detailed than a Methods section of a scientific paper, since you should make explicit the structure of your analyses.
  2. A concise presentation of your results, including tables and figures as appropriate. Because of the strict space limits, you will need to be selective in the analyses you present.
  3. A brief discussion, summarising your main conclusions and discussing any potential sources of error or bias.

Criteria for grading

5 Excellent:

An outstanding report which clearly answers the question, shows indepth understanding of the analysis and is well-explained.

4 Very good:

A thorough analysis, with all relevant information reported.

3 Good:

Sound analysis, but some relevant points are omitted and/or the presentation lacks clarity.

2 Satisfactory:

Basic understanding of major points is shown, but some errors in the analysis or interpretation, or muddled presentation.

1 Unsatisfactory:

Inadequate analysis and lack of understanding shown.

0 Very poor:

Serious lack of understanding shown: inappropriate analysis used.

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