Validity is the degree to which a research design accurately measures what it is intended to measure, while excluding possible alternate explanations for the findings.

Designing a valid measure is more complicated than it might seem. In usability tests, for example, testers frequently measure how long it takes each user to complete a given task. But, that measure may not always be an accurate reflection of the usability of the interface.

Here’s an illustration. In the past, I’ve seen reports based on using web server log data to measure how long, on average, users took to add something to an online shopping cart and proceed through a checkout process. While it might seem like “time to completion of task” is a useful measure of usability, it doesn’t account for other possible explanations for the users’ behavior. Some users may be distracted by cross-selling opportunities and leave the shopping cart task to browse for other items. Other users may be distracted by other tasks in their environment, such as a phone call or other interruption. Some users may have opened other browser windows to research prices with different vendors before completing their purchases. All of these possibilities could lead to longer “time-to-completion of task” measures, but that doesn’t make the shopping cart interface any less usable! So averaging the time-to-completion data contained in the log file is statistically valid, but completely useless in this case. Since other possible explanations for the results besides poor usability were not excluded or accounted for, the time-to-completion measure doesn’t actually measure the overall usability of the shopping cart interface.

Reliability refers to the extent to which repeated measurements using the research design under similar conditions produce the same results.

However, reliability does not guarantee validity. It’s entirely possible to design a study that is reliable, but still does not result in valid data. In the example above, the time-to-completion average remained consistent in the server log data from week to week. This makes the measure reliable, but it does not increase its validity, since it is not an accurate measure of the shopping cart’s usability.

Sampling refers to the practice of selecting a representative subset of study participants from the total population of interest.

Researchers use sampling because studying everyone in a population is usually far too impractical and expensive. Theoretically, sampling allows the researcher to generalize their findings from the subset onto the larger population. For example, sampling a subset of potential users out of the total user population and observing how they navigate a particular interface allows one to generalize the behavior of the sample onto the larger population with a specified degree of certainty. Most people are familiar with random sampling — one in which every person in the population has an equal chance of being selected for the sample. However, there are several other types of sampling methods available to the researcher, each with its own level of precision and difficulty.

The sampling method chosen for any usability study should balance the costs of the method against the desired level of precision needed for the study. A “convenience sample” of co-workers recruited from down the hall might be inexpensive and easy, but it may not provide the most appropriate level of validity for a particular project. For example, if these testers are already familiar with the application under development, the test may result in findings that are not representative of the actual user population. Hence, the validity of a usability study based on a convenience sample may be questionable. We recommend different sampling techniques depending on the characteristics of the user population of interest and the risk levels associated with a specific project. Typically, we use methods that ensure that test participants reflect the relevant characteristics of the system’s intended user population.

Generally speaking, the larger the size of the sample, the more reliable the study will be. But, remember that reliability does not directly relate to validity! The costs of increasing the sample size (and the reliability of the study) should be balanced against the increase’s potential return on investment. Consider how much that extra reliability is worth to you. In scientific studies, the acceptable probability level for making a claim is usually set very high — at 95% or 99%. Put into straightforward terms, this means that studies are designed so that “based on our observations of the sample, we can predict that the general population will act the same way at least 95% of the time.”

It’s not difficult to design or conduct usability tests that deliver statistically valid results. In fact, at Classic System Solutions, we are familiar with a variety of research designs and probability sampling methods. However, we have found that designing usability studies to ensure a high level of statistical significance is rarely cost effective for our clients. Instead, we have found that using a small number of test participants allows us to significantly reduce risk for the majority of our clients, while maximizing the return on their investment.

The risks associated with the user interface of a web page or software application are tremendous. We know, for example, that usability problems in a system will significantly decrease end user efficiency and increase support costs. End users also tend to resist adopting systems with poor usability, so the competitor with the easier user interface gets the edge in the marketplace. And, several studies have shown that incorporating usability checks into the development process is far less expensive than redesigning a system to make it more usable late in the development cycle, or even after an initial launch. Any of these factors may significantly reduce the return on your company’s investment in developing software or web applications.

While it’s important to manage risk in the business world, it’s also important to minimize costs. While it’s not difficult to design usability tests that deliver statistically valid results, the cost of conducting these tests can be prohibitive due to the large sample sizes required, and the benefits marginal. Since we recommend that you conduct usability tests early in the design process to identify areas that need improvement, it’s usually much cheaper to just fix the problems identified by a small group of testers than it would be to statistically confirm the results of the tests with a larger population of users.

Furthermore, research has demonstrated that the number of usability problems, found in a design, levels off significantly after the first six testers. The first five test participants typically discover approximately 85% of the usability problems in a task; but it might take another ten testers to find the remaining 15%. Consequently, we find that the smaller sample size is a more cost-effective choice for most development projects, even if it does not engender statistical validity.

Plus, you can use the money you saved on running that enormous study to run additional test cycles later in the development process. This not only allows you to validate the design choices made to fix the initial problems, but also ensures that no additional usability problems are introduced by the new designs.