The null hypothesis refers to the default prediction in research that no significant statistical relationship exists between two variables or two sets of data. The assumption in the null hypothesis is that any measured differences that result from interaction of the two variables are simply the result of randomness.

Six Sigma practitioners typically consider the null hypothesis along with the alternative hypothesis. The alternative hypothesis proposes that interaction between two variables does produce outcomes that are not the result of randomness. It also states that it’s possible to prove this statistically significant relationship through data analysis.

In other words, the alternative hypothesis “rejects the null” and shows evidence of correlation between the two sets of data.

Much of the statistical analysis done in Six Sigma seeks to reject the null hypothesis. That is, it looks to find relationships between variables and identify what action (or lack of action) is leading to unwanted variation and errors. By identifying these relationships, teams can focus on the root cause of a problem.

A Null Hypothesis Example From Sports

The null hypothesis seeks to eliminate assumptions. Rather than look at outcomes and assume an interaction between variables is causing the outcome, the null hypothesis requires the default position that the result is random. Testing then must determine whether it’s possible to reject the null hypothesis and prove some type of correlation.

A simple example involves a hitter on a baseball team. A person might attend many games over the years and make the observation that batters tend to get on base at a higher rate during day games than night games.

To determine if that is true, data analysts would look at two sets of data: one showing the on-base percentage of hitters during day games and one showing the on-base percentage for hitters during night games. This is exactly what researchers at the University of California-Berkeley decided to do.

The null hypothesis calls for a default prediction that any differences in day game vs. night game hitting statistics are generated by randomness. In the case of Berkeley researchers, they  constructed a histogram of on-base percentages for batters and other statistical models using 2019 season statistics. They could not find enough statistical evidence to determine that time of day has an impact on a hitter’s ability to make on base.

In this case, data analysis failed to reject the null. The person who saw differences in how often hitters reached based on the games they attended simply witnessed the outcome of statistical randomness in a complex game.

An Example of Null Hypothesis From Business

The steps for a basic hypothesis testing involve identifying the question, determining the significance, choosing the test, interpreting the results and making a decision. In businesses that adhere to Six Sigma, it takes on increased significance because rejecting the null and finding correlation helps to identify challenges that need addressing.

Finding the null hypothesis is relatively easy. Simply take a question and rephrase it as a statement to start with the null.

  • Q: Do more website visitors click on the “find out more” when it is blue rather than red or green?
  • Null: The color of the “find out more” button has no impact on how often people click on it.

 

  • Q: Does the number of people on a project team impact whether the team achieves its goal?
  • Null: The number of people on a project team does not impact the team’s ability to achieve goals.

For the first question, data research would analyze the interaction of one data set (the number of clicks) vs other data sets (the number of clicks when the button is red, blue and green). In the second question, the number of team members would be compared to each team’s rate of success (research in this area suggests it’s five or less). The results would either reject the null or find no meaningful relationship between the two variables.

A null hypothesis also provides a starting place to determine basic business issues such as the impact of different materials on the quality of the final product, the types of advertising that lead to better sales or the impact of social media postings on website traffic.

Null hypothesis also helps investors make financial decisions, according to Investopedia. For example, they will look at return rates of mutual funds over many years to determine what variables do or do not have an impact on annual returns.