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Dr. Moataza Mahmoud Abdel Wahab

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More accuracy i.e. Can Do A Better Job of Data Collection ... The population to be studied/ to which the investigator wants to generalize his results ... – PowerPoint PPT presentation

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Title: Dr. Moataza Mahmoud Abdel Wahab


1
Sampling techniques sample size
  • Dr. Moataza Mahmoud Abdel Wahab
  • Lecturer of Biostatistics
  • High Institute of Public Health
  • University of Alexandria
  • moatazamahmoud_at_yahoo.com

2
Important statistical terms
  • Population
  • a set which includes all
  • measurements of interest
  • to the researcher
  • (The collection of all responses,
    measurements, or counts that are of interest)
  • Sample
  • A subset of the population

3
Why sampling?
  • Get information about large populations
  • Less costs
  • Less field time
  • More accuracy i.e. Can Do A Better Job of Data
    Collection
  • When its impossible to study the whole population

4
  • Target Population
  • The population to be studied/ to which the
    investigator wants to generalize his results
  • Sampling Unit
  • smallest unit from which sample can be selected
  • Sampling frame
  • List of all the sampling units from which sample
    is drawn
  • Sampling scheme
  • Method of selecting sampling units from sampling
    frame

5
Types of sampling
  • Non-probability samples
  • Probability samples

6
Non probability samples
  • Convenience samples (ease of access)
  • sample is selected from elements of a population
    that are easily accessible
  • Snowball sampling (friend of friend.etc.)
  • Purposive sampling (judgemental)
  • You chose who you think should be in the study
  • Quota sample

7
Non probability samples

Probability of being chosen is unknown Cheaper-
but unable to generalise potential for bias
8
Probability samples
  • Random sampling
  • Each subject has a known probability of being
    selected
  • Allows application of statistical sampling theory
    to results to
  • Generalise
  • Test hypotheses

9
Conclusions
  • Probability samples are the best
  • Ensure
  • Representativeness
  • Precision

10
Methods used in probability samples
  • Simple random sampling
  • Systematic sampling
  • Stratified sampling
  • Multi-stage sampling
  • Cluster sampling

11
Simple random sampling
12
Table of random numbers
  • 6 8 4 2 5 7 9 5 4 1 2 5 6 3 2 1 4 0
  • 5 8 2 0 3 2 1 5 4 7 8 5 9 6 2 0 2 4
  • 3 6 2 3 3 3 2 5 4 7 8 9 1 2 0 3 2 5
  • 9 8 5 2 6 3 0 1 7 4 2 4 5 0 3 6 8 6

13
Systematic sampling
  • Sampling fraction
  • Ratio between sample size and population size

14
Systematic sampling
15
Cluster sampling
Cluster a group of sampling units close to each
other i.e. crowding together in the same area or
neighborhood
16
Cluster sampling
Section 2
Section 1
Section 3
Section 5
Section 4
17
  • Stratified sampling
  • Multi-stage sampling

18
Errors in sample
  • Systematic error (or bias)
  • Inaccurate response (information bias)
  • Selection bias
  • Sampling error (random error)

19
Type 1 error
  • The probability of finding a difference with our
    sample compared to population, and there really
    isnt one.
  • Known as the a (or type 1 error)
  • Usually set at 5 (or 0.05)

20
Type 2 error
  • The probability of not finding a difference that
    actually exists between our sample compared to
    the population
  • Known as the ß (or type 2 error)
  • Power is (1- ß) and is usually 80

21
Sample size
Quantitative
Qualitative
22
  • Problem 1
  • A study is to be performed to determine a certain
    parameter in a community. From a previous study a
    sd of 46 was obtained.
  • If a sample error of up to 4 is to be accepted.
    How many subjects should be included in this
    study at 99 level of confidence?

23
Answer
24
Problem 2
  • A study is to be done to determine effect of 2
    drugs (A and B) on blood glucose level. From
    previous studies using those drugs, Sd of BGL of
    8 and 12 g/dl were obtained respectively.
  • A significant level of 95 and a power of 90 is
    required to detect a mean difference between the
    two groups of 3 g/dl. How many subjects should be
    include in each group?

25
Answer
26
Problem 3
  • It was desired to estimate proportion of anaemic
    children in a certain preparatory school. In a
    similar study at another school a proportion of
    30 was detected.
  • Compute the minimal sample size required at a
    confidence limit of 95 and accepting a
    difference of up to 4 of the true population.

27
Answer
28
Problem 4
  • In previous studies, percentage of hypertensives
    among Diabetics was 70 and among non diabetics
    was 40 in a certain community.
  • A researcher wants to perform a comparative study
    for hypertension among diabetics and
    non-diabetics at a confidence limit 95 and power
    80, What is the minimal sample to be taken from
    each group with 4 accepted difference of true
    value?

29
Answer
30
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31
Thank
You
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