4.sampling design
DESCRIPTION
Contains research methodology contents might be useful to medical and paramedical students pursuing researchTRANSCRIPT
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Sampling Design
D.A. Asir John Samuel, BSc (Psy), MPT (Neuro Paed), MAc, DYScEd,
C/BLS, FAGE
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Basic definitions
• Population
- Collection of all the units that are of interest
to the investigator
• Sample
- Representative part of population
• Sampling
- Technique of selecting a representative group
from a population Dr. Asir John Samuel (PT), Lecturer, ACP 2
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Why ?
• Only feasible method for collecting information
• Reduces demands on resources (time, finance,.)
• Results obtained more quickly
• Better accuracy of collected data
• Ethically acceptable
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Steps in sampling design
Target population
Study population
Sample
Study participation Dr. Asir John Samuel (PT), Lecturer, ACP 4
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Characteristic of good sample design
• True representation of population
• May result in small sampling error
• Each member in population should get an
opportunity of being selected
• Systematic bias can be controlled in a better way
• Results should be capable of being extrapolated Dr. Asir John Samuel (PT), Lecturer, ACP 5
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Types of sample design
• Probability/Random sampling
- Selection of subjects are according to any
predicted chance of probability
• Non-probability/non-random sampling
- Does not depend on any chance of predecided
probability
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Types of sample design
Sample design
Random sampling
Simple Stratified Systematic Cluster Multistage
Non-random sampling
convenience Quota Judgment
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Simple random sampling
• Equal and independent chance or probability
of drawing each unit
• Take sampling population
• Need listing of all sampling units (sampling
frame)
• Number all units
• Randomly draw units Dr. Asir John Samuel (PT), Lecturer, ACP 8
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How to ensure randomness?
• Lottery method
• Table of random numbers
- e.g. Tippett’s series
- Fisher and Yates series
- Kendall and Smith series
- Rand corporation series Dr. Asir John Samuel (PT), Lecturer, ACP 9
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SRS - Merits
• No personal bias
• Easy to assess the accuracy
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SRS - Demerits
• Need a complete catalogue of universe
• Large size sample
• Widely dispersed
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Stratified Random Sampling
• Used for heterogeneous population
• Population is divided into homogeneous
groups (strata), according to a characteristic of
interest (e.g. sex, religion, location)
• Then a simple random sample is selected from
each stratum
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SRs - Merits
• More representative
• Greater accuracy
• Can acquire information about whole
population and individual strata
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SRs - Demerits
• Careful stratification
• Random selection in each stratum
• Time consuming
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Systematic Sampling
• Sampling units are selected in a systematic
way, that is, every Kth unit in the population is
selected
• First divide the population size by the,
required sample size (sampling fraction). Let
the sampling fraction be K
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Systematic Sampling
• Select a unit at random from the first K units
and thereafter every Kth unit is selected
• If, N=1200
• And n=60
• Then, SF=20
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SS - Merits
• Simple and convenient
• Less time and work
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SS - Demerits
• Need complete list of units
• Periodicity
• Less representation
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Cluster Sampling
• The sampling units are groups or clusters
• The population is divided into clusters, and a
sample of clusters are selected randomly
• All the units in the selected clusters are then
examined or studied
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Cluster Sampling
• It is always assumed that the individual items
within each cluster are representation of
population
• E.g. District, wards, schools, industries
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CS - Merits
• Saving of travelling time and consequent
reduction in cost
• Cuts down on the cost of preparing the
sampling frame
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CS - Demerits
• Units close to each other may be very similar
and so, less likely to represent the whole
population
• Larger sampling error than simple random
sampling
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Multistage Sampling
• Selection is done in stages until final sampling
units are arrived
• At first stage, Random sampling of large sized
sampling units are selected, from the selected
1st stage sampling units another sampling
units of smaller sampling units are selected,
randomly Dr. Asir John Samuel (PT), Lecturer, ACP 23
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Multistage Sampling
• Continue until the final sampling units are
selected
• E.g. Few states – District – Taulk
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MS - Merits
• Cut down the cost of preparing the sampling
frame
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MS - Demerits
• Sampling error is increased compared to
simple random sampling
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Quota Sampling
• Interviewers are requested to find cases with
particular types of people to interview
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Judgment (Purposive Sampling)
• Researcher attempts to obtain sample that
appear to be representative of the population
selected by the researcher subjectively
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Convenience Sampling
• Sampling comprises subject who are simply
avail in a convenient way to the researcher
• No randomness and likelihood of bias is high
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Snowball Sampling
• Investigators start with a few subjects and
then recruit more via word of mouth from the
original participants
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Merits
• Easy
• Low cost
• Limited time
• Total list population
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Demerits
• Selection bias
• Sample is not representation of population
• doesn’t allow generalization
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Sample Size
Determination
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p-value
• Probability of getting a minimal difference of
what has observed is due to chance
• Probability that the difference of at least as
large as those found in the data would have
occurred by chance
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Hypothesis
• Alternate hypothesis (HA)
- Statement predict that a difference or
relationship b/w groups will be demonstrated
• Null hypothesis (H0)
- Researcher anticipate “no difference” or “no
relationship”
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Decision for 5% LOS
• If p-value <0.05, then data is against null
hypothesis
• If p-value ≥0.05, then data favours null
hypothesis
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Type I & II errors
Possible states of Null Hypothesis
Possible actions on
Null Hypothesis
True False
Accept Correct Action
Type II error
Reject Type I error
Correct Action
Prob (Type I error) – α (LoS) Prob (Type II error) – β 1-β – power of test
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Z values
Z 0.05 – 1.96 – 95%
Z 0.10 – 1.282 – 90%
Z 0.20 – 0.84 – 80%
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Comparison of 2 means
n= 2 [(Zα+Zβ)s/d]²
Zα – LoS
Zβ – power of study
s – pooled SD of the two sample
d – clinically significant difference
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Eg. for Comparison of 2 means
• A RCT to study the effect of BP reduction. One group received a control diet and other-test diet. What would be the sample size in order to provide the study with power of 90% to detect a difference in sys. BP of 2 mm Hg b/w two groups at 5% LoS? The SD of sys. BP is observed to be 6 mmHg.
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Estimating proportion
n = Z α² P (1-P) / d²
P – proportion of event in population
d – acceptable margin of error in estimating the true population proportion
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Eg. Estimating proportion
• To determine the prevalence of navicular drop in ACL injured population by anticipating of 15% with acceptable margin of error is 3%
= (1.96)²(0.15)(0.85) / (0.03)²
= 544.2
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Estimating mean
n = (Zα σ / d)²
σ – anticipated SD of population
d – acceptable margin of error in estimating true population mean
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Eg. Estimating mean
• To determine the mean no. of days to ambulate pt undergoing stroke rehabilation among stroke pts. Where anticipated SD of days are 60 and acceptable margin of error is 20 days
n = (1.96 x 60/20)²
n = (5.88)² = 34.6
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Comparison of 2 proportions
n = (Zα √2PQ + Zβ√P1Q1+P2Q2)²/(P1-P2)²
P = P1+P2/2 Q = 1-P
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Eg. Comparison of 2 proportions
• To see whether there is any sig. difference in percentage of strength increase after 4 wks of intervention b/w a new technique and standard one
• Standard one – 70% (P1)
• New technique – 75% (P2)
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