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Equivalence Testing for Quality Analysis (Part I): What are You Trying to Prove?

With more options, come more decisions.

With equivalence testing, you now have more statistical tools to test a sample mean against target value or another sample mean.

Equivalence testing is extensively used in the biomedical field. Pharmaceutical manufacturers often need to test whether the biological activity of a generic drug is equivalent to that of a brand name drug that has already been through the regulatory approval process.

But in the field of quality improvement, why might you want to use an equivalence test instead of a standard t-test?

Interpreting Hypothesis Tests: A Common Pitfall

Suppose a manufacturer finds a new supplier that offers a less expensive material that could be substituted for a costly material currently used in the production process. This new material is supposed to be just as good as the material currently used. It should not make the product too pliable nor too rigid.

To make sure the substitution doesn’t negatively impact quality, an analyst collects two random samples from the production process (which is stable): one using the new material and one using the current material.

The analyst then uses a standard 2-sample t-test (Stat > Basic Statistics > 2-Sample t in Minitab Statistical Software) to assess whether the mean pliability of the product is the same using both materials:

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Two-Sample T-Test and CI: Current, New

Two-sample T for Current vs New
             N    Mean   StDev    SE Mean
Current   9   34.092   0.261     0.087
New      10   33.971  0.581     0.18

Difference = μ (Current) - μ (New)
Estimate for difference:  0.121
95% CI for difference:  (-0.322, 0.564)
T-Test of difference = 0 (vs ≠): T-Value = 0.60  P-Value = 0.562  DF = 12
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Because the p-value is not less than the alpha level (0.05), the analyst concludes that the means do not differ. Based on these results, the company switches suppliers for the material, confident that statistical analysis has proven that they can save money with the new material without compromising the quality of their product.

The test results make everyone happy. High-fives. Group hugs. Popping champagne corks. There’s only one minor problem.

Their statistical analysis didn’t really prove that the means are the same.

Consider Where to Place the Burden of Proof

In hypothesis testing, H1 is the alternative hypothesis that requires the burden of proof. Usually, the alternative hypothesis is what you’re hoping to prove or demonstrate. When you perform a standard 2-sample t-test, you’re really asking: “Do I have enough evidence to prove, beyond a reasonable doubt (your alpha level), that the population means are different?”

To do that, the hypotheses are set up as follows:

If the p-value is less than alpha, you conclude that the means significantly differ. But if the p-value is not less than alpha, you haven’t proven that the means are equal. You just don’t have enough evidence to prove that they’re not equal.

The absence of evidence for a statement is not proof of its converse. If you don’t have sufficient evidence to claim that A is true, you haven’t proven that A is false.

Equivalence tests were specifically developed to address this issue. In a 2-sample equivalence test, the null and alternative hypotheses are reversed from a standard 2-sample t test.

This switches the burden of proof for the test. It also reverses the ramification of incorrectly assuming (H0) for the test.

Case in Point: The Presumption of Innocence vs. Guilt

This rough analogy may help illustrate the concept.

In the court of law, the burden of proof rests on proving guilt. The suspect is presumed innocent (H0), until proven guilty (H1). In the news media, the burden of proof is often reversed: The suspect is presumed guilty (H0), until proven innocent (H1).

Shifting the burden of proof can yield different conclusions. That’s why the news media often express outrage when a suspect who is presumed to be guilty is let go because there was not sufficient evidence to prove the suspect’s guilt in the courtroom. As long as news media and the courtroom reverse their null and alternative hypotheses, they’ll sometimes draw different conclusions based on the same evidence.

Why do they set up their hypotheses differently in the first place? Because each seems to have a different idea of what’s a worse error to make. The judicial system believes the worse error is to convict an innocent person, rather than let a guilty person go free. The news media seem to believe the contrary. (Maybe because the presumption of guilt sells more papers than presumption of innocence?)

When the Burden of Proof Shifts, the Conclusion May Change

Back to our quality analyst in the first example. To avoid losing customers, the company would rather err by assuming that the quality was not the same using the cheaper material--when it actually was--than err by assuming it was the same, when it actually was not.

To more rigorously demonstrate that the means are the same, the analyst performs a 2-sample equivalence test (Stat > Equivalence Tests > Two Sample).

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Equivalence Test: Mean(New) - Mean(Current)

Test
Null hypothesis:         Difference ≤ -0.4 or Difference ≥ 0.4
Alternative hypothesis:  -0.4 < Difference < 0.4
α level:                 0.05

Null Hypothesis    DF  T-Value  P-Value
Difference ≤ -0.4  12   1.3717    0.098
Difference ≥ 0.4   12  -2.5646    0.012

The greater of the two P-Values is 0.098. Cannot claim equivalence.
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Using the equivalence test on the same data, the results now indicate that there isn't sufficient evidence to claim that the means are the same. The company cannot be confident that product quality will not suffer if they substitute the less expensive material. By using an equivalence test, the company has raised the bar for evaluating a possible shift in the process mean.

Note: If you look at the above output, you'll see another way that the equivalence test differs from a standard t-test. Two one-sided t-tests are used to test the null hypothesis. In addition, the test uses a zone of equivalence that defines what size difference between the means you consider to be practically insignificant. We’ll look at that in more detail in my next post.

Quick Summary

To choose between an equivalence test and a standard t-test, consider what you hope to prove or demonstrate. Whatever you hope to prove true should be set up as the alternative hypothesis for the test and require the burden of proof. Whatever you deem to be the less harmful incorrect assumption to make should be the null hypothesis. If you’re trying to rigorously prove that two means are equal, or that a mean equals a target value, you may want to use an equivalence test rather than a standard t-test.

Comments

Name: Stan Alekman • Tuesday, April 8, 2014

OK. In your example, there is not enough evidence in the std 2-sample t-test to prove that Ha is false. But what if we used a large enough sample size that is capable of detecting a difference we consider important, the minimum detectable difference, with a Power of 90%. Then there is a 10% chance of failing to detect a difference that size. If we accept Ho with the understanding that differences below the minimum detectable difference of the test is of no concern (ie equivalent), why can't we use the ordinary 2-sample t-test? In this case we have the evidence, don't we, to conclude Ho is true?


Name: Patrick • Thursday, April 10, 2014

Excellent point. I was hoping someone would raise the issue of power for a standard t-test.

You're absolutely right. If the power of a 2-sample t-test is 90%, and you fail to reject H0, there is only a 10% chance that the test did not detect a significant difference between the population means, when such a difference actually does exist. IF, for your given application, you have that high of power for your test and IF you're comfortable with a 10% chance that the test missed a significant difference that does exist in the population, due to random error, then you might well be satisfied using a standard t-test, feeling comfortable that you have enough evidence to assume that H0 is true (the population means are the same) if the test does not detect a significant difference.

However, at a significance level of 0.05, note that you still have a 5% higher chance of making a Type 2 error than you do of making a Type 1 error, even with such high power. If the Type 2 error has extremely serious consequences compared to the Type 1 error, you'd still be better off using an equivalence test to swap the hypotheses and thereby reduce the chances of the more serious error.

More to the point, a power of 90% is often very hard to achieve in practice. In fact, 80% is often considered the benchmark, and even THAT much power is often not obtained in the real world, due to contraints of time and budget. Many times researchers who perform prospective power analyses, as they should, are crestfallen because they simply do not have the resources to collect as many samples as they need to to achieve adequate power. So in this imperfect world, often the power achieved will be 70%, 60% or even less--and the chance of Type 2 error in real-world applications that use standard hypothesis tests can wind up being more than 30%!

This is one reason equivalence tests were developed in the first place. It is often not feasible in bioequivalence trials to round up enough participants for a study to achieve adequate power to sufficiently reduce the Type 2 error rate for a standard hypothesis test. (A Type 2 error in this case would mean they would claim a generic version of a drug is equivalent to an already approved and tested drug--when it was not. A very serious error that would mean people using the generic drug would not be getting an equally effective medication!) Hence, the need to switch null and alternative hypotheses so that they could instead take advantage of a much lower 0.05 Type I error rate when demonstrating equivalence. That gives them only 5% chance of rejecting the null and concluding equivalence, when in fact the population means for the 2 drugs actually differ significantly.

Look for these issues addressed more clearly and concretely an upcoming post.

Thanks for your astute comment! Much appreciated!!


Name: Wayne G. Fischer, PhD • Tuesday, April 22, 2014

Hmmm...I'm wondering if, for something as important as switching raw materials while maintaining customer satisfaction, a better way to decide "equivalence" is to use an appropriate DoE to see if the new material really *is* equivalent to the old in its performance - *throughout* the region of operability and not just at a single set of conditions.

After all, all processes vary - especially so in manufacturing - and adjustments are made to compensate accordingly (a form of "robustness")...so I'd want a *robust* assessment of "equivalence."


Name: Patrick • Wednesday, April 23, 2014

I completely agree with you and I’m glad you raised this point.In coming up with a scenario whereby the consequence of a Type 2 error is much more serious than a Type 1 error, I may have overshot the mark here. I should clarify that the main point of this post isn’t intended to be “use an equivalence to determine whether to switch suppliers” but rather “if you’re using a standard t-test to try to prove the process mean is equal to a target value or another mean, and the repercussions of a Type 2 error are much more serious than a Type 1 error, consider using an equivalence test instead.”

As you suggest, a t-test alone—a standard t-test or an equivalence test—may not in and of itself provide sufficient evidence for switching suppliers in many cases. Obviously, it would depend on the process, the product, the resources available, how many changeable factors are involved in operating conditions, and so on. Given your point that “all processes vary”, one should ideally run many other analyses before making any significant process change: measurement systems analysis to ensure the measuring system is accurate and consistent, control charts to ensure the process mean and variation are stable, capability analysis to evaluate how well specifications are being met, DOE to evaluate the response(s) over the range of operating conditions…

Thanks for your comment!


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