Choosing study subjects - PowerPoint PPT Presentation

1 / 26
About This Presentation
Title:

Choosing study subjects

Description:

1. To be able to define the population to which you seek to ... '20% of patients attending Dermatology OPD have eczema' = proportion (not prevalence) ... – PowerPoint PPT presentation

Number of Views:69
Avg rating:3.0/5.0
Slides: 27
Provided by: academi8
Category:

less

Transcript and Presenter's Notes

Title: Choosing study subjects


1
Choosing study subjects
  • Prof. Rodney Ehrlich
  • School of Public Health and Family Medicine

2
Learning Objectives
  • 1. To be able to define the population to which
    you seek to generalise your results
  • 2. To be able to identify your sampling frame
    (or recruitment pool)
  • 3. To be able to choose a sampling strategy (or
    randomisation strategy)
  • 4. To understand the distinction between random
    sampling error and systematic error (bias) and
    how to minimise these

3
Population
  • Statistical term any coherent group to which
    you wish to generalise your results
  • Typically share some characteristic(s) of common
    interest
  • Geography (e.g. Cape Town)
  • Health relevant characteristic (Tik users)
  • May be defined by health services use ?
    fundamental distinction between patient and
    population based studies.

4
What are your patients representative of?
  • Compared to population at large, i.e. with
    respect to population level characteristics?
  • Compared to patients with same condition or
    diagnosis who are not hospitalised (or who do
    not attend that health service).

5
Selecting the population you want to study
  • Inclusion and exclusion criteria
  • Demographic age, gender, race, socioeconomic
    status
  • Clinical risk factor or outcome of interest,
    stable, no aggravating factors,
    contraindications, etc.
  • Administrative availability vs difficulty
  • Ethical ability to consent, record access, etc.
  • Always trade off between efficiency and
    generalisability

6
Digression on terminology prevalence and
incidence in hospital patients
  • Population measures, not health service
    measures.
  • 20 of patients attending Dermatology OPD have
    eczema proportion (not prevalence)
  • 10 of admissions to ICU were for acute MI
    proportion (not incidence).

7
Why sample?
  • Cant afford to study whole population
  • No need to study whole population, as sampling is
    an efficient (i.e. lower cost) way to get the
    same information

8
Which sampling strategy?
  • Depends on
  • study design (in turn dependent on the question)
  • availability of subjects/information on subjects
  • resources

9
Different types of sampling
  • Convenience (or availability) sampling
  • Probability sampling

10
Convenience sampling
  • Haphazard no logic other than availability of
    subjects
  • Volunteer
  • Consecutive

11
Probability sampling
  • Simple random
  • Stratified random
  • Systematic random
  • Cluster random
  • Important when want truly representative
    estimates of population ? mainly in descriptive
    studies or surveys
  • Basis of statistical procedures

12
Simple random sampling
  • Starts with sampling frame (e.g. list, archive)
    representing population
  • Random precise statistical term each subject
    has known chance of being selected
  • Use table or programme or other procedure to
    generate random numbers

13
Stratified random sampling
  • Divide sampling frame into strata (i.e. subjects
    in strata share common characteristic)
  • Random sampling within each stratum
  • Can oversample some strata (remember to reweight
    if want an estimate for the whole)

14
Systematic random sampling
  • Sampling frame is consecutive (e.g. records in
    a filing system, or patients arriving)
  • Use random number start
  • Then choose every nth subject, where n sampling
    interval (e.g. every 10th subject), until sample
    size is reached

15
Cluster random sampling
  • Mainly for population/community surveys
  • Take advantage of naturally occurring
    administrative units (e.g. community health
    centres, schools, blocks of houses). Each unit is
    a cluster
  • Can also create clusters, as in Extended
    Programme on Immunisation (EPI)
  • Have to take cluster effect into account in the
    sample size calculation and the analysis
  • Cluster sampling A sampling method in which each
    unit selected is a group of persons rather than
    an individual - Last

16
Sampling error
  • If random with respect to outcome of interest
    random error
  • If systematic or biased with respect to
  • measurement interest systematic error or
    bias

17
Random sampling error
  • As likely to be in one direction as another
  • Expected value (e.g. mean) is unbiased
  • ? wider confidence interval or larger p-value
    (i.e. greater uncertainty)
  • Control of random error increase sample size

18
Systematic sampling error / bias
  • In one direction more than another
  • Expected value (e.g. mean or ) is biased
  • Minimise systematic error by improving selection
    process and measurement instruments

19
Types of systematic error / bias
  • Selection bias - arises before data collection
  • Measurement bias arises during data collection

20
Randomisation
  • Form of assignment, not sampling
  • NOT the same as random sampling
  • Purpose is to distribute potential confounders
    equally between groups

21
Exercise 1
  • Discuss how patients at the UCT linked
    hospitals/facilities may differ from
  • a. the general population of the Western Cape
  • b. patients from Cape Town with the same
    diagnosis and condition not attending these
    facilities.

22
Exercise 2
  • You want to do a case-control study of the
    possible causes of childhood leukaemia . Discuss
  • a. how you would select your cases
  • b. how you would select controls (without
    leukaemia).

23
Exercise 3
  • You do a telephone survey of patients waiting for
    elective orthopaedic surgery. You are aware of
    selection bias arising from the fact that some
    patients do not have telephone access. Also, the
    response rate (those available and willing to be
    interviewed) among those that do have telephone
    numbers is 65.
  • a. Would increasing the sample size help to
    minimise the potential selection biases?
  • b. What other ways could help to reduce these
    biases?

24
Exercise 4
  • You are planning a study of health
    professionals attitudes to performing elective
    terminations of pregnancy within the provincial
    administration health services.
  • Devise a sampling strategy.

25
Exercise 5
  • You are planning to conduct a randomised
    controlled trial of a new oral anticoagulant for
    chronic atrial fibrillation.
  • a. How would you recruit patients? Is
    sampling a consideration here?
  • b. To whom would you like to generalise
    your findings?
  • c. What would your inclusion and exclusion
    criteria be?

26
Exercise 6
  • You are planning to test the performance of
    a new rapid screening test for prostatic specific
    antigen against the standard laboratory test.
  • a. How would you recruit patients? Is
    sampling a consideration here?
  • b. To whom would you like to generalise
    your findings?
  • c. What would your inclusion and exclusion
    criteria be?
Write a Comment
User Comments (0)
About PowerShow.com