Multiple Indicator Cluster Surveys Data Processing Workshop Sample Weights MICS Data Processing Workshop
Dec 15, 2015
Multiple Indicator Cluster SurveysData Processing Workshop
Sample Weights
MICS Data Processing Workshop
What are sample weights ?
• Sample weight: a statistical correction factor used to correct for imperfections in the sample that might lead to bias:– Unequal probabilities of selection– Non-response
• Constant sampling weight: self-weighting sample
Self-weighting sample
• Constant sampling weight: self-weighting sample– Stratum level (e.g., urban and rural within
region)– National level: overall self-weighting sample
(almost inexistent in household surveys)
Self-weighting sample
• Advantages– Equally representative for every unit – Reduced sampling errors
• Disadvantages:– Difficult for survey management (e.g., to distribute
the work-load) because of the variant sample take by PSU
– Difficult to control the expected sample size
Self-weighting sample
• Disadvantages– Self-weighting is not exact because of the
rounding of the sample takes and this will bring bias in the survey estimation
• In most MICS surveys, if not all, samples are not self-weighting. Therefore, sample weights must be used for reporting national estimates
Example - Sample Weights• For example, the weights for North and West regions
(Popstan) – North region 10,000/500 = 20– West region 10,000/250 = 40
• In North region, each household selected represents 20 households in that region – same figure is 40 in West
• Overall, every household selected in Popstan represents 26.6667 households (20,000/750)
Example - Sample Weights
• In other words, relative to a proportional selection (should be 375 households selected from each region), more households have been selected from North, less have been selected from West
• This has to be “compensated” by using sample weights during analysis to re-calibrate the sample to the national level
Example - Sample Weights
• In our example let’s assume that: – 25 percent of households in North use improved
water sources– 75 percent of households in West use improved
water sources• If the sample was selected proportionally (375
households from each region), then our survey estimate would be – ((375 * 0.25) + (375 * 0.75)) / 750 = 0.50
Example - Sample Weights
• If we do not weight, then our national estimate will be– ((500 * 0.25) + (250 * 0.75)) / 750 = 0.417– Because, we have over-sampled a region (North
region) where use of improved water sources is less
• We need to calculate sample weights to “correct” this situation
Example - Sample Weights
• If we assigned a weight of 20 to each household in North, and 40 to each household in West, this would do the trick
(500 * 20 * 0.25) + (250 * 40 * 0.75)-----------------------------------------------
(500 * 20) + (250 * 40)
= 0.50
Example - Sample Weights
• This is fine, but SPSS tables would show 20,000 households as the denominator
• We do not want this• So, we normalize the weights• We calibrate (normalize) them so that the
average of the weights in the data set is equal to 1
Example - Sample Weights• The normalized weight for the North region is
calculated as (10000/500)/(20000/750) = 0.75• And for the West region, (10000/250)/(20000/750)
= 1.5
When we calculate the national use of improved water sources by using normalized weights,
(500 * 0.75 * 0.25) + (250 * 1.5 * 0.75) 375-------------------------------------------------- = -----(500 * 0.75) + (250 * 1.5) 750
Sample weights
• Based on the design of the sample, there are two (common) approaches to calculating weights:– Each cluster has a unique sample weight
(weights.xls)– Each stratum has a unique sample weight
(weights_alt.xls)• We have templates for both. You will need to
work with your sampling expert to see which one you will use
Sample Weights Objects
• weights.xls– spreadsheet that calculates weights
• weights_table.sps– SPSS program that provides input data for spreadsheet
• weights.sps– SPSS program that defines structure of spreadsheet’s output
• weights_merge.sps– SPSS program that merges weights onto the MICS data files
Calculating sample weights
• The spreadsheet weights.xls is used to calculate the sample weights
• It has two worksheets, calculations and output. – The calculations worksheet performs the
calculations– The output worksheet contains only the sample
weights and a list of cluster numbers; format useful for reading the data into SPSS
• Weights calculation template
Calculating and adding sample weights
• weights_table.sps produces data needed for calculating the sample weights
• weights_merge.sps adds the appropriate sample weights to the analysis files
Steps in calculating sample weights
• The process of calculating sample weights and adding them to your analysis files can be broken down into six steps
Steps in calculating sample weights
Step 1• Adjust the number of rows in the calculations
and output worksheets so that there is one row per cluster in your survey. After you have added or deleted rows, be sure to check that doing so did not affect the totals row in the calculations worksheet
Steps in calculating sample weights
Step 2• Enter required information for columns B to F
and for columns H and I
Steps in calculating sample weights
Step 3• Update the definition of strata (or domains) on lines
3 through 10 of the program weights_table.sps• The standard programs assume that strata are
formed by all combinations of area (that is, urban and rural) and region and that there are four regions (the program should be modified to reflect the strata or domains in use in your sample)
Steps in calculating sample weights
Step 4• Execute the program weights_table.sps.
Steps in calculating sample weights
Step 5• Copy the information in the table and
paste it into the calculations worksheet of weights.xls
• When you complete this step, weights.xls will automatically calculate the sample weights
Steps in calculating sample weights
Step 6• Execute the program weights_merge.sps• Once you have completed the sixth step,
be sure to check the output list for error messages and to open the analysis files and confirm that the weights have been properly merged