The Four Cornerstones of Survey Measurement: Part 1
10/30/17 / Matt Bruce
Part One: Precision and Accuracy
Years ago, I worked in an environmental lab where I measured the amount of silt in water samples by forcing the water through a filter, drying the filters in an oven, then weighing the filters on a calibrated scale. I followed very specific procedures to ensure the results were precise, accurate, reliable, and valid; the cornerstones of scientific measurement.
As a social-science researcher today, I still use precision, accuracy, reliability, and validity as indicators of good survey measurement. The ability of decision makers to draw useful conclusions and make confident data-driven decisions from a survey depends greatly on these indicators.
To introduce these concepts, I’ll use the metaphor of figuring out how to travel from one destination to another, say from your house to a new restaurant you want to try. How would you find your way there? You probably wouldn’t use a desktop globe to guide you, it’s not precise enough. You probably wouldn’t use a map drawn in the 1600’s, it wouldn’t be accurate. You probably shouldn’t ask a friend who has a horrible memory or sense of direction, their help would not be reliable. What you would likely do is “Google It,” which is a valid way most of us get directions these days.
This two-part blog will unpack the meaning within these indicators. Let’s start with precision and accuracy. Part-two will cover reliability and validity.
Precision refers to the granularity of data and estimates. Data from an open-ended question that asked how many cigarettes someone smoked in the past 24 hours would be more precise than data from a similar closed-ended question that listed a handful of categories, such as 0, 1-5, 6-10, 11-15, 16 or more. The open-ended data would be more precise because it would be more specific, more detailed. High precision is desirable, all things being equal, but there are often “costs” associated with increasing precisions, such as increased time to take a survey, that might not outweigh the benefit of greater precision.
Accuracy refers to the degree that the data are true. If someone who smoked 15 cigarettes in the past 24 hours gave the answer ‘5’ to the open-ended survey question, the data generated would be precise but not accurate. There are many possible reasons for this inaccuracy. Maybe the respondent truly believed they only smoked five cigarettes in the past 24 hours, or maybe they said five because that’s what they thought the researcher wanted to hear. Twenty-four hours may have been too long of a time span to remember all the cigarettes they smoked, or maybe they simply misread the question. If they had answered “between 1 and 20,” the data would have been accurate, because it was true, but it wouldn’t have been very precise.
Many times, an increase in precision can result in a decrease in accuracy, and vice-versa. Decision makers can be confident in accurate data, but it might not be useful. Precise data typically give researchers more utility and flexibility, especially in analysis. But what good is flexible data if there is little confidence in its accuracy. Good researchers will strive for an appropriate balance between precision and accuracy, based on the research goals and desired outcomes.
Now that we have a better understanding of precision and accuracy, the second blog in this series will explore reliability and validity.