# Unit 3: Sample Size, Sampling Methods, Duration and Frequency of Sampling #3-3-1.

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Unit 3: Sample Size, Sampling Methods, Duration and Frequency of Sampling #3-3-1

Warm Up Questions: Instructions  Take five minutes now to try the Unit 3 warm up questions in your manual.  Please do not compare answers with other participants.  Your answers will not be collected or graded.  We will review your answers at the end of the unit. #3-3-2

What You Will Learn  By the end of this unit you should be able to: © explain the process of sampling © determine who is eligible for a sample © choose the appropriate sampling scheme for specific situations © plan for the duration and frequency of sampling to be conducted #3-3-3

The Sampling Process  Appropriate and consistent sampling methods ensure that sero-surveys are representative of populations in which they are conducted.  Representative samples resemble the true population.  They ensure that surveys are consistent from year to year, and are consistent among sites. #3-3-4

Components of the Sampling Process  Sample size, or number of individuals to include in the sample  Sampling scheme, or procedures for choosing individuals to be included in the sample  Duration and frequency of sampling, how long to sample and how often to sample #3-3-5

Sample Size  The number of subjects included in a sentinel surveillance sample is guided by the need to: © determine trends in HIV prevalence over time © identify sub-populations at high risk for infection  Sample size needs to be large enough to be able to detect the difference between two prevalence estimates.  Statistically, this is referred to as the margin of error (for example, ±3%). #3-3-6

Sample Size, Cont.  The standard statistical approach for determining the sample size requires: © estimate of HIV prevalence © the margin of error considered acceptable © the level of confidence desired  Typically, sample size calculations are done by a country’s national AIDS control programme  Minimum sample sizes should be calculated for each sentinel site for each survey period. #3-3-7

Calculating Sample Size with Epi Info  The STATCALC feature of Epi Info™ software provides a user-friendly sample size calculator for setting specific target sample sizes.  Learn more about Epi Info™ and download the software for free at this site: http://www.cdc.gov/epiinfo #3-3-8

Determining Sample Size in Practice  A minimum sample size between 250 and 400 patients per site is recommended. © A margin of error of ±3% and 95% statistical confidence can be reached. © This is the average sample size that can be obtained in a typical clinic over 8 to 12 weeks. © This provides sufficient statistical power to determine trends in HIV prevalence over a 3-year period. #3-3-9

Figure 3.1. Formula to Determine Sample Size  z  is a factor that corresponds to the desired confidence interval (for a 95% confidence level, z  = 1.96)  P is the expected proportion of patients with the outcome of interest (in this case, P = expected HIV prevalence).  W is the width of the interval, for example the width for a margin of error of +/- 3% is 0.06. #3-3-10

Sample Size Rules of Thumb  The closer the estimated prevalence is to 50% the larger the sample size that will be required to achieve the same confidence interval width.  The sample size needed gets larger as the desired margin of error gets smaller.  A sample size must be practical to achieve in the course of a few months. #3-3-11

Sampling Schemes  There are three basic sampling schemes that are used at sentinel sites: © consecutive sampling © systematic sampling © simple random sampling  Each scheme has advantages & disadvantages. #3-3-12

Consecutive Sampling  In consecutive sampling, select every patient that meets the inclusion criteria until you get to the required sample size or the survey period is over.  Example: Include every ANC patient that meets the inclusion criteria that comes into the ANC clinic during the month of January. #3-3-13

Systematic Sampling  In systematic sampling, make a list of every patient who meets the inclusion criteria. Select every n th person from list until required sample size is reached or survey period is over.  Example: Make a list of all ANC attendees that meet the inclusion criteria, and select every third patient until you have 256 participants or the survey period is over. #3-3-14

Simple Random Sampling  In simple random sampling, generate a list of random numbers. Match numbers to a list of patients who meet study criteria until the required sample size is met.  Lists of random numbers can be taken from random number tables, or can be generated by computer or other methods. #3-3-15

Table 3.1. Advantages and Disadvantages of Consecutive Sampling Advantages  relatively easy to employ  less opportunity for intentional or unintentional manipulation by clinic staff or errors due to confusion Disadvantages  not based on randomisation  may be variations in who is seen at ANCs at different times of the year (for example during the rainy season women may have problems coming from rural areas to the clinic) #3-3-16

Table 3.1. Advantages and Disadvantages of Systematic Sampling Advantages  more likely than consecutive sampling to produce representative sample if done correctly Disadvantages  more difficult to do correctly  requires more attention to procedural details  non-random selection of the first patient or failure to sample every n th patient will make sample non- random  method may also take longer to fill sample size #3-3-17

Table 3.1. Advantages and Disadvantages of Simple Random Sampling Advantages  most likely to produce a true sample of the total population if done correctly Disadvantages  complex, prone to errors and confusion, and may take longer to conduct  requires a census or numbered list of potential survey participants #3-3-18

Duration of Surveys  HIV prevalence in a given area is likely to change over time.  Ideally, HIV prevalence is measured at a single point in time, in a ‘point estimate.’  Practically, point estimates are usually impossible to obtain.  In order to best approximate a point estimate, sampling time period should be limited. #3-3-19

Duration of Surveys, Cont.  Give health workers in clinics precise dates on when to begin and end time-period for specimen collection.  Many countries set period at or below 10 weeks.  If clinic volume is low, sampling period may be longer.  One requirement for inclusion of a site is the ability to fill sample size requirements in allotted sampling period. #3-3-20

Frequency of Surveys  Select the sampling period as the same time each year (for example, 1 August through 30 September) and in the same sites.  This ensures data are comparable from year to year, and place to place.  To allow sufficient time for collation, analysis, interpretation and report writing, sero-surveys should be repeated every 1-2 years. #3-3-21

Frequency of Surveys, Cont.  More frequent surveys do not usually contribute additional information that is useful for public health decisions.  Among the most important considerations in determining the frequency of surveys is the availability of sufficient resources. #3-3-22

Inclusion and Exclusion Criteria  Inclusion criteria: characteristics required in study participants, in order to be considered for sample.  Exclusion criteria: characteristics of patients who should be excluded from sample, but who would otherwise be eligible.  Goals of setting criteria: © minimising potential survey biases © avoiding inclusion of a person multiple times #3-3-23

Table 3.2. Sample Inclusion and Exclusion Criteria for ANC Surveys Inclusion Criteria  age (only women 15 to 49 years old should be eligible for the sample) Exclusion Criteria  has previously attended ANC during current pregnancy  attendance at another ANC #3-3-24

Warm Up Review  Take a few minutes now to look back at your answers to the warm up questions at the beginning of the unit.  Make any changes you want to.  We will discuss the questions and answers in a few minutes. #3-3-25

Answers to Warm Up Questions 1.Which of the following is a reason to have inclusion and exclusion criteria? a.Include as few participants as possible in the survey b.avoiding including the same person multiple times c.both of the above #3-3-26

Answers to Warm Up Questions, Cont. 2.True or false? The goal of sampling is to use the data from a sample of a population to estimate the HIV prevalence in the larger population. True #3-3-27

Answers to Warm Up Questions, Cont. 3.Which of the following is a decision that needs to be made at the beginning of a sampling procedure? a.the sample size b.the sampling scheme c.the frequency of sampling d.all of the above #3-3-28

Answers to Warm Up Questions, Cont. 4.True or false? As much as possible, the sampling period should be limited, in order to compare HIV prevalence over time. True #3-3-29

Answers to Warm Up Questions, Cont. _c_ consecutive _a_ systematic _b_ simple random a)randomly selects the initial patient who meets inclusion criteria, and then selects every nth eligible patient thereafter b)uses a computer or other method to generate random numbers that identify patients to be included in the sample c)samples every patient that meets the inclusion criteria until the required sample size is achieved 5. Match each sampling scheme with its description: #3-3-30

Answers to Warm Up Questions, Cont. 6.Which of the above schemes is the most simple logistically, and best reduces the likelihood of selection bias? Consecutive #3-3-31

Answers to Warm Up Questions, Cont. 7.True or false? All subjects at the sentinel site who meet the inclusion criteria during the sampling period should be included in the survey. False #3-3-32

Answers to Warm Up Questions, Cont. 8.True or False? When surveys are repeated, they should be carried out in different sites from the initial survey and during a different time of the year. This helps give a clear picture of the epidemic’s scope. False #3-3-33

Small Group Discussion: Instructions  Get into small groups to discuss these questions.  Choose a speaker for your group who will report back to the class.  Take 15 minutes for this exercise. #3-3-34

Small Group Reports  Select one member from your group to present your answers.  Discuss with the rest of the class. #3-3-35

Case Study: Instructions  Try this case study individually.  We’ll discuss the answers in class. #3-3-36

Case Study Review  Follow along as we go over the case study in class.  Discuss your answers with the rest of the class. #3-3-37

Questions, Process Check  Do you have any questions on the information we just covered?  Are you happy with how we worked on Unit 3?  Do you want to try something different that will help the group? #3-3-38

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