# Duality: Confidence intervals and the severity of tests

A question came up in our seminar today about how to understand the duality between a simple one-sided test and a lower limit (LL) of a corresponding 1-sided confidence interval estimate. This is also a good route to SEV (i.e., severity). Here’s a quick answer:

Consider our favorite test of the mean of a Normal distribution with n iid samples, and known standard deviation σ: test T+. This time let:

H0: µ ≤  0 against H1: µ >  0 , and let σ= 1.

Nothing of interest to the logic changes if s.d. is estimated as is more typical. If σ = 1, n = 25, (σ/ √n) = .2.

The (1 – α) confidence interval (CI) corresponding to test T+ is that µ > the (1 – α) LL:

µ > M – ca(1/ √n ).

where M represents the statistic, usually written X-bar, the sample mean. For example,

M – 2.5(1/ √n )

is the generic lower limit (LL) of a 99% CI. The impressive thing is that this holds regardless of the true value of µ. If, for any M you assert:

µ > M – ca(1/ √n ),

your assertions will be correct 99% of the time. [Once the data are in hand, M takes the value of a particular sample mean. Without quantifiers, this is a little imprecise.]

Now for the duality between CIs and tests. How does it work?

Put aside for the moment our fixed hypothesis of interest; just retain the form of test T+. Keeping the s.d. of 1, and n = 25, suppose we have observed M = .6.

Consider the question: For what value of µ0 would M = .6 be the 2.5 s.d. cut-off (in test T+)? That is, for what value of µ0 would an observed mean of .6 exceed µby 2.5 s.d.s? (Or again, for what value of µ0 would our observation reach a p-value of .01 in test T+?)

Clearly, the answer is in testing H0: µ ≤  .1 against H1: µ >  .1.

The corresponding .99 lower limit of the one-sided confidence interval would be:

[.1 < µ , infinity]

The duality with tests says that these are the µ values (in the given model and test) that would not be statistically significant at the .01 level, had they been the ones tested in T+. For example:

H0: µ ≤  .15 would not be rejected, nor would H0: µ ≤  .2, H0: µ ≤  .25 and so on. That’s because the observed M is not statistically significantly greater (at the .01 level) than any of the µ values in the interval. Since this is continuous, it does not matter if the cut-off is just at .1 or values greater than .1.

On the other hand, a test hypothesis of H0: µ ≤  .09 would be rejected by M = .6; as would µ ≤  .08, µ ≤  .07…. H0: µ ≤  0, and so on. Using significance test language again, the observed M is statistically significantly greater than all these values (p-level smaller than .01), and at smaller and smaller levels of significance.

Under the supposition that the data were generated from a world where H0: µ ≤  .1 against µ >.1,  at least 99% of the time a larger M than was observed would occur.

The test was so incapable of having produced so large a value of M as .6, were µ less than the 99% CI lower bound, that we argue there is an indication (if not full blown evidence) that µ > .1.

We are assuming these values are “audited”, and the assumptions of the model permit the computations to be approximately valid. Following Fisher, evidence of an experimental effect requires more than a single, isolated significant result, but let us say that is satisfied.

The severity with which µ > .1 “passes” the test with this result M = .6 (in test T+)  is ~ .99.

SEV( µ > .1, test T+, M = .6) = P(M < .6; µ =.1) = P( Z < (.6 – .1)/.2)=

P(Z < 2.5) = .99.

Here’s a little chart for this example:

Duality between LL of 1-sided confidence intervals and a fixed outcome M = .6 of test T+: H0: µ ≤  µ0 vs H1: µ >  µ0. σ = 1, n = 25, (σ/ √n) = .2. These computations are approximate.

 Were µ no greater than The capability of T+ to produce M as large as .6 is _ µ is the 1-sided LL with level _ Claim C SEV associated with C .1 .01 .99 (µ > .1) .99 .2 .025 .975 (µ > .2) .975 .3 .07 .93 (µ > .3) .93 .4 .16 .84 (µ > .4) .84 .5 .3 .7 (µ > .5) .7 .6 .5 .5 (µ > .6) .5 .7 .69 .31 (µ > .7) .31

In all these cases, the test had fairly low capability to produce M as large at .6–the largest it gets is .69. I’ll consider what the test is more capable of doing in another post. Note that: as the capability increases, the corresponding confidence level decreases.

Categories: confidence intervals and tests, Phil6334