Sampling techniques & sample

Sampling techniques & sample

Dr. Moataza Mahmoud Abdel Wahab

Lecturer of Biostatistics

High Institute of Public Health

University of Alexandria

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

Why sampling?

Get information about large populations

ê  Less costs

ê  Less field time

é  More accuracy i.e. Can Do A Better Job of Data Collection

î  When it’s impossible to study the whole population

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

Types of sampling

n  Non-probability samples

n  Probability samples

Non probability samples

Probability of being chosen is unknown Cheaper- but unable to generalise potential for bias

Ø  Convenience samples (ease of access)

Sample is selected from elements of a population that are easily accessible

Ø  Snowball sampling (friend of friend….etc.)

Ø  Purposive sampling (judgemental)

Ø  You chose who you think should be in the study

Ø  Quota sample

Probability samples

n  Random sampling

n  Each subject has a known probability of being selected

n  Allows application of statistical sampling theory to results to:

n  Generalise

n  Test hypotheses

Conclusions

n  Probability samples are the best

n  Ensure

n  Representativeness

n  Precision

Methods used in probability samples

Ø  Simple random sampling

Ø  Systematic sampling

Ø  Stratified sampling

Ø  Multi-stage sampling

Ø  Cluster sampling

Simple random sampling

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

Systematic sampling

Sampling fraction Ratio between sample size and population size

Cluster sampling Cluster: a group of sampling units close to each other i.e. crowding together in the same area or neighborhood

n  Stratified sampling

n  Multi-stage sampling

Errors in sample

Ø  Systematic error (or bias) Inaccurate response   (information bias) Selection bias

Ø  Sampling error (random error)

Type 1 error

n  The probability of finding a difference with our sample compared to population, and there really isn’t one….

n  Known as the α (or “type 1 error”)

n  Usually set at 5% (or 0.05)

Type 2 error

n  The probability of not finding a difference that actually exists between our sample compared to the population…

n  Known as the β (or “type 2 error”)

n  Power is (1- β) and is usually 80%

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?

Answer

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?

Answer

 

n  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.

Answer

n  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?

Answer