# One- vs. two-tailed inference¶

Note

This discussion is applicable only to one- and two-sample tests involving the t statistic including t tests and regression.

It is inapplicable to test statistics whos values are always greater than zero (i.e. F, X^{2} and T^{2} statistics).

By default **spm1d** conducts two-tailed inference for one- and two-sample tests.

To force one-tailed inference use the keyword *two_tailed*, as in the following example.

Import necessary packages:

```
>>> import numpy as np
>>> import spm1d
```

Generate ten smooth, random 1D continua sampled at 100 nodes:

```
>>> np.random.seed(0)
>>> Y = np.random.randn(10,100) +0.21
>>> Y = spm1d.util.smooth(Y, fwhm=10)
```

Conduct one-sample t test, with one- then two-tailed inference:

```
>>> t = spm1d.stats.ttest(Y)
>>> ti_1 = t.inference(alpha=0.05, two_tailed=False) #one-tailed inference
>>> ti_2 = t.inference(alpha=0.05, two_tailed=True) #two-tailed inference
```

From the results (below) we see that the one-tailed results reach the *alpha* = 0.05 threshold, and that the two-tailed results do not.
The reasons is:

Note

The one-tailed threshold specifies the height that an SPM{t}’s **maximum value** would reach in only *alpha%* of many repeated experiments, if those SPM{t}s were generated by smooth, random (Gaussian) continua. The two-tailed threshold contrastingly specifies the height that an SPM{t}’s **maximum absolute value** would reach in *alpha%* of many repeated experiments.

In other words, it is more likely that an SPM{t}’s **maximum absolute value** will reach an arbitrary threshold than its **maximum value**.
Protecting against this increased likelihood, and retaining **alpha** is fortunately simple:

Note

A one-tailed threshold at *alpha* is equivalent to a two-tailed threshold at (*alpha* / 2).

Specifying “*two_tailed* = *True*” achieves two goals:

The critical test statistic threshold is set according to (

*alpha*/ 2).Probability values are corrected to the higher threshold, so that an SPM{t} which just touches the threshold will, by definition, yield a

*p*value of*alpha*.

Warning

Choosing one- vs. two-tailed inference must be done in an *a priori* manner, before conducting an experiment. If you are unsure which to choose, then you should conduct two-tailed inference. More specifically, if you don’t have a specific hypothesis regarding the direction of a particular effect (e.g. Group A has a greater value than Group B), then it is most objective to expect that the effects could be in either direction.

(Source code, png, hires.png, pdf)