Survey & Sampling 2014
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Transcript of Survey & Sampling 2014
Survey and Sampling
Assistant Professor: Ahmed S. Ishtiaque
ULAB
What is a Survey?
• A systematic method of collecting information from a sample of people from a population about a set of questions for the purposes of describing some attributes of the population
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Features of a Survey
• Information is collected from a sample of population
• By asking questions using a standardized questionnaire
• Produce statistics (quantitative or numerical description about some aspects of the study population)
• Generalizable to the whole population
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Types of Survey (timeline based)• Cross sectional
– collect information on outcome of interest and population variables
– at one particular time
• Panel/ Cohort – repeated administration of a questionnaire to a
“panel‟ of households/ group of people sharing common experience/ characteristics
– variables are measured on the same units over time– can add extra module to answer a new RQ
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Types of Survey (content based)
• Descriptive Survey: Description of a population, certain behavior, life style, disease prevalence etc.
• Analytical Survey: Hypothesis driven charaterized by identifying association or linakage between/amongst the variables.
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1. Establish the goal of the project - What do you want to know
2. Select the sample - Whom will you interview
3. Choose interview methodology - How will you interview
4. Specify variables of interest – What information do you want
5. Create questionnaire (i.e. instrument) - What will you ask
Steps of survey
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6. Pre-test questionnaire – Are your questions clear, understandable and follow logical patterns
7. Conduct interviews - Ask the questions
8. Data editing and entry – Check for consistency
9. Data analysis – Answer research questions
Steps of survey (contd..)
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• Think of whether your survey aims at collecting information at the individual or at the household level
• Set up inclusion and exclusion criteria for sample
Sample Determination (Whom will you interview)
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Choose interview methodology – How will you interview?
Self-administered questionnaire
Face-to-face interviews
Preserves confidentiality – limits risk of providing expected answers
Interferes with confidentiality – higher risk of providing expected answers
Requires extremely simple & well-structured questions
Allows interviewer to work with interviewee on more complex questions
High probability of low response rate/missing values
Probable introduction of bias depending on how questions are phrased
Hybrid interview strategies: Phone, computer-assisted & email
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» Written Survey
• Oral Survey
– Electronic Survey
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Choose interview methodology (How will you interview)
Self-administered questionnaire or face-to-face interview?
Choice depends on:• Study question• Study setting
• Structure of questionnaire• Resources available
• Research team preference
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Specify variables of interest (What information do you want)
• Always write down list of variables of interest before drafting the questionnaire
• Check for balance and coherence between variables
• Develop a question or a set of questions for each variable in the list
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Specify variables of interest (contd..)(What information do you want )
Variables Questions
Age What is your age?
Profession What is your job?
Marital status What is your marital status?
This strategyalso facilitates
division ofquestionsin sections
Strengths of a Survey
• Useful in describing characteristics of a large population
• Very large samples are feasible, making results statistically significant
• Standardized questions make measurement more precise by enforcing uniform definitions upon the participants.
• High reliability is easy to obtain14
Weaknesses of a Survey
• Not a good method for research on sensitive topics
• Require the initial study design (the tool and administration of the tool) to remain unchanged throughout the data collection.
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Errors in Survey
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Characteristics of Population
Sample of population members who
answer questions
Answers respondent give
Issue:How well answers
measure characteristics to
be described?
Issue:How closely sample responding mirrors
population?
Related to:Sampling
Related to:Questionnaire design & administration
Why Sampling? • Often difficult or impossible to study total
population• Studying a part may provide dependable
information• Important to select a part or subgroup of the
population in a way that the information obtained is generalizable to the total population
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Sampling
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• This part or subgroup is called sample and the process of selecting the sample is sampling
• Why sample?– Resources (time, money) and workload– Gives results with known accuracy that can be
calculated mathematically
What Sampling?Population
Sample
Using data to say something (make an inference) with confidence, about a whole (population) based
on the study of a only a few (sample).
Sampling Frame
Sampling Process
What you want to talk
about
What you actually
observe in the data
Inference
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The Sampling Process
Define the Population
Determine the Sampling Frame
Select Sampling Technique(s)
Determine the Sample Size
Execute the Sampling Process
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Population• Sometimes referred as ‘Universe’
– The entirety. – All the members/elements within a specific
category. – Size and characteristics depend on type of
study
• To define a population – ask yourself: – What am I studying?– To whom or what does my study result apply
to? (related to generalization)21
Sample
Study population: the population that is actually listed in your sampling frame
Sampling frame: listing of study population from which you'll draw your sample
Sample: a part of the whole Sampling: process of selection of the
required number of sampling units from a defined population
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An example
• What percentage of Women (20-55) in Dhaka Metropolitan city in Bangladesh were diagnosed with uterine cancer in 2000?–Study Population?–Sampling Frame?
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• Study Population?• Women (20-55) in Metropolitan Dhaka in
2000 who do not have a history of hysterectomy.
• Sampling Frame?• List of all women (20-55) in Metropolitan
Dhaka in 2000 who do not have a history of hysterectomy.
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Classification of Sampling Techniques
Sampling Techniques
NonprobabilitySampling Techniques
ProbabilitySampling Techniques
ConvenienceSampling
PurposiveSampling
QuotaSampling
SnowballSampling
SystematicSampling
StratifiedSampling
ClusterSampling
Multi-stageSampling
Simple RandomSampling 25
Probability Sampling
• A sampling method that gives each unit in the population a known, non-zero equal chance of being selected is called a probability sampling method
• No unit receives preferences over the other• No units is left out intentionally
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Type of Probability Sampling
Simple random sampling Systematic sampling Stratified sampling Cluster sampling Multi-stage sampling
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Simple Random Sampling
Each group member has the same probability of being selected
Example: Selecting balls from a basket Can be done by using sampling frame and
random table Best applicable to homogeneous population Example ?
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Simple Random Sampling• The most basic sampling design.• If you can make a complete list of your target
population then you can use simple random sampling
• The idea is to assign a number to each of the units in a population and then use a random number generator of some sort to choose the respondents for the analysis.
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Systematic Sampling An estimate is made of expected total number of
units in the study population Divide this number by required sample size The resulting number is the sampling interval (n) Every nth unit is selected till the total sample size is
drawn
Example ?
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Stratified Sampling
When a population is heterogeneous Study population is first divided into homogeneous
groups or classes called strata The choice of stratification variables depends on
the variables that matter for responses.
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Stratified Sampling In most public health research, natural candidates
are race, income, education, gender, location, etc. Simple random sampling is performed in each
strata Allocation of samples among the strata can be
proportional to the size of the strata
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Stratified Sampling• Proportionate: when sampling units in the
strata are selected proportional to their representation in the source population
• Disproportionate: deliberately increasing the size of sampling units selected from a particular strata so they represent a disproportionate figure in the sample compared to the source population
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Cluster Sampling Groups of individuals are sampling units rather
than individuals Population is first divided into groups or
clusters A part of these clusters are then selected using
simple random sampling or systematic sampling
Saves money and time
Example ?
34
Multistage Sampling When the total population is large and diverse Sample selection is carried out in several
stages Different sampling units at different stages or
levels Units are selected using simple random
sampling or systematic sampling
Example ?
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Non-probability Sampling
• The sampling units are selected as convenient to the researcher
• Has a greater chance of giving biased results
• Example ?
36
Non-probability Sampling: types
• Convenience sampling: A convenience sample is simply one where the units that are selected for inclusion in the sample are the easiest to access.
• Purposive sampling: Sampling technique that rely on the judgment of the researcher when it comes to selecting the units that are to be studied.
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Type of Non-probability Sampling
• Snowball sampling: Snowball sampling is particularly appropriate when the population you are interested in is hidden and/or hard-to-reach.
• Quota sampling: the researcher decides in advance on certain key characteristics which s/he will use to stratify the sample.
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Selecting a sampling method
• Population to be studied– Size/geographical distribution– Heterogeneity with respect to variable
• Availability of list of sampling units• Level of precision required• Resources available
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Differences between two types of sampling
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Probability sampling Non-probability sampling
Always for quantitative research method
Usually for qualitative research method
Sampling units have known probability of being selected
No such thing
Involves statistical analysis Does not involve statistics.
Results can be generalized Results are not intended to be generalized
Sample size calculation is involved
Sample size calculation is not involved
Sampling Error
• Samples may be different from population – Choosing sample frame– Process of selecting sample– Failure to collect answers from everyone
(non-response)
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An example:Bangladesh Maternal Mortality and Health Care
Survey (BMMS) 20102nd nationally representative survey to:• provide national estimates of the maternal mortality
ratio (MMR) in Bangladesh
• identify causes of maternal deaths among adult women;
• understand antenatal, delivery and post-natal care
Sample Size: around 175,000 households
Sampling method: multi-stage cluster sampling
Used a three stage sampling procedure • First stage: wards in urban and unions in rural
were used as the primary sampling units (PSUs)
• Second stage: selected two mohallas in each ward and two mouzas in each union
• Third stage: Each selected mohalla and mouza was segmented into clusters and one of these was selected from each selected mohalla and mouza
Unions
Segment
Household
Mouzas
Rural Areas
Wards
Segment
Household
Mohallas
Urban Areas
Sample Sizes Selected
Domains Clusters HouseholdsUrban 654 42510
Other Urban 488 31720Rural 1566 101790Total 2708 176020
Response Rates: Households
Urban Rural Total0
25
50
75
100 98.6 98.9 98.898.2 98.6 98.4
2001 2010
Response Rates Women
Urban Rural Total0
25
50
75
100 96.6 97.3 97.296.9 97.7 97.3
2001 2010