The goal of collecting data is to provide evidence for or against a hypothesis. Take a moment to think about what ‘evidence’ is – most researchers I ask can’t come up with a good answer. For example, researchers sometimes think p-values are evidence, but p-values are only correlated with evidence.

Evidence in science is necessarily relative. When data is more likely assuming one model is true (e.g., a null model) compared to another model (e.g., the alternative model), we can say the model provides evidence for the null compared to the alternative hypothesis. P-values only give you the probability of the data under one model – what you need for evidence is the relative likelihood of two models.

Bayesian and likelihood approaches should be used when you want to talk about evidence, and here I’ll use a very simplistic likelihood model where we compare the relative likelihood of a significant result when the null hypothesis is true (i.e., making a Type 1 error) with the relative likelihood of a significant result when the alternative hypothesis is true (i.e., *not* making a Type 2 error).

Let’s assume we have a ‘methodological fetishist’ (Ellemers, 2013) who is adamant about controlling their alpha level at 5%, and who observes a significant result. Let’s further assume this person performed a study with 80% power, and that the null hypothesis and alternative hypothesis are equally (50%) likely. The outcome of the study has a 2.5% probability of being a false positive (a 50% probability that the null hypothesis is true, multiplied by a 5% probability of a Type 1 error), and a 40% probability of being a true positive (a 50% probability that the alternative hypothesis is true, multiplied by an 80% probability of finding a significant effect).

The relative evidence for H1 versus H0 is 0.40/0.025 = 16. In other words, based on the observed data, and a model for the null and a model for the alternative hypothesis, it is 16 times more likely that the alternative hypothesis is true than that the null hypothesis is true. For educational purposes, this is fine – for statistical analyses, you would use formal likelihood or Bayesian analyses.

Now let’s assume you agree that providing evidence is a very important reason for collecting data in an empirical science (another goal of data collection is estimation – but I’ll focus on hypothesis testing here). We can now ask ourselves what the effect of changing the Type 1 error or the Type 2 error (1-power) is on the strength of our evidence. And let’s agree that we will conclude that whichever error impacts the strength of our evidence the most, is the most important error to control. Deal?

We can plot the relative likelihood (the probability a significant result is a true positive, compared to a false positive) assuming H0 and H1 are equally likely, for all levels of power, and for all alpha levels. If we do this, we get the plot below:

So when is the evidence in our data the strongest? Not surprisingly, this happens when both types of errors are low: the alpha level is low, and the power is high (or the Type 2 error rate is low). That is why statisticians recommend low alpha levels and high power. Note that the shape of the plot remains the same regardless of the relative likelihood H1 or H0 is true, but when H1 and H0 are not equally likely (e.g., H0 is 90% likely to be true, and H1 is 10% likely to be true) the scale on the likelihood ratio axis increases or decreases.

Now for the main point in this blog post: we can see that an increase in the Type 2 error rate (or a reduction in power) reduces the evidence in our data, but it does so relatively slowly. However, we can also see that an increase in the Type 1 error rate (e.g., as a consequence of multiple comparisons without controlling for the Type 1 error rate) quickly reduces the evidence in our data. Royall (1997)recommends that likelihood ratios of 8 or higher provide moderate evidence, and likelihood ratios of 32 or higher provide strong evidence. Below 8, the evidence is weak and not very convincing.

If we calculate the likelihood ratio for alpha = 0.05, and power from 1 to 0.1 in steps of 0.1, we get the following likelihood ratios: 20, 18, 16, 14, 12, 10, 8, 6, 4, 2. With 80% power, we get the likelihood ratio of 16 we calculated above, but even 40% power leaves us with a likelihood ratio of 8, or moderate evidence (see the figure above). If we calculate the likelihood ratio for power = 0.8 and alpha levels from 0.05 to 0.5 in steps of 0.05, we get the following likelihood ratios: 16, 8, 5.3, 4, 3.2, 2.67, 2.29, ,2, 1.78, 1.6. An alpha level of 0.1 still yields moderate evidence (assuming power is high enough!) but further inflation makes the evidence in the study very weak.

To conclude: Type 1 error rate inflation quickly destroys the evidence in your data, whereas Type 2 error inflation does so less severely.

Type 1 error control is important if we care about evidence. Although I agree with Fiedler, Kutzner, and Kreuger (2012) that a Type 2 error is also very important to prevent, you simply can not ignore Type 1 error control if you care about evidence. Type 1 error control is more important than Type 2 error control, because inflating Type 1 errors will very quickly leave you with evidence that is too weak to be convincing support for your hypothesis, while inflating Type 2 errors will do so more slowly. By all means, control Type 2 errors – but not at the expense of Type 1 errors.

I want to end by pointing out that Type 1 and Type 2 error control is not a matter of ‘either-or’. Mediocre statistics textbooks like to point out that controlling the alpha level (or Type 1 error rate) comes at the expense of the beta (Type 2) error, and vice-versa, sometimes using the horrible seesaw metaphor below:

Image from: http://www.statisticsfromatoz.com/blog/statistics-tip-of-the-week-the-alpha-and-beta-error-seesaw

But this is only true if the sample size is fixed. If you want to reduce both errors, you simply need to increase your sample size, and you can make Type 1 errors and Type 2 errors are small as you want, and contribute extremely strong evidence when you collect data.