Most researchers prefer building a sample from a single source. In many cases, however, getting a truly representative sample from a single source is becoming more difficult. Survey Sampling International (SSI) has used a blended sample approach of panels, web traffic, and aligned interest groups, and has found the resulting quality of the data is higher than with a single source sample.

Using a blended sample source creates two benefits:
(1) It helps capture the opinions of people who would not otherwise join panels; and 

(2) It increases heterogeneity. As the breadth of sources increases, however, it is important to identify the unique biases of each of those sources and control for it in order to ensure high sample quality. The only way to achieve this balance is to understand where the bias is coming from. By using a panel exclusively, for example, you might eliminate individuals with valuable opinions who just aren’t willing to commit to joining the panel.

Researchers should also make sure their samples are consistent and predictable. Studies indicate that controlling just for demographics and other traditional balancing factors does not always account for the variations created by the distinct characteristics of different sample sources.
Demographic quotas may work, but only if the selected stratification relates directly to the questionnaire topic. Comparing sources to external benchmarks can improve consistency as well, but often those benchmarks are not readily available.

SSI’s research on variance between data sources indicates that psychographic and neurographic variables have a greater capacity to influence variance between diverse sources than traditional demographic variables have. Even still, these variables do not account for all the possible variance, so researchers must continue testing in order to ensure consistency within the blended sampling method.

SSI offers the following suggestions for creating a blended sample:

■ Consider including calibration questions—Look for existing external benchmarks for your survey topic.

■ Understand the sample blending techniques used to create your sample—Tell your sample provider what kind of source smoothing and quality control methods are being used.

■ Know your sources—Ask your sample provider how source quality is being maintained.

■ Plan ahead—Incorporate blending into the sample plan from the start.
■ Ensure that respondents are satisfied with the research experience—Be aware that significantly high nonresponse and noncompletion rates can introduce bias as well.

Questions 

1. Beyond the variables discussed, can you think of any others that might be relevant when creating a blended sample?

2. Do you think a blended sample would be useful, and if so, would you be inclined to try it? Are there any situations in which you would think a single-source sample would be more effective? Why?