# Statistics Ground Zero/Significance

## SignificanceEdit

In statistical testing we deal in probabilities. To ask our research question in a statistically testable way is to ask

*If the null hypothesis is true, how likely is it that I would observe the data that I have collected?*

Put slightly more technically

*The p value represents the probability of seeing data this extreme if the***null hypothesis**were true

We set a threshold, most commonly 99% or 95%, meaning that we acknowledge that we might be misled into rejecting the null hypothesis 1% or 5% of the time respectively. *P* must fall below this threshold for us to reject the null hypothesis. That is to say *p* must be less than 0.01 or less than 0.05 (the inverse of 99% and 95% expressed as decimals).

This value, the **p value**, is said to determine whether the outcome of a test is **significant** or not. If the outcome is significant then the null hypothesis is rejected.

### Choosing a testEdit

Very often people find step three above - choosing the correct test - the most difficult, but if we know what we want to do and something about the nature of our data it is not so very difficult. The following table covers a surprisingly large number of common cases.

Question | Measure of Dependent Variable | Two Variables or Groups | More than Two Variables or Groups | Parametric | Non-parametric |
---|---|---|---|---|---|

Is there an association? | Nominal | Yes | ^{[1]} |
Chi-square | |

Is there an association? | Ordinal | Two | Spearman's correlation coefficient (with an indication of strength) | ||

Is there an association? | Scalar | Two | Pearson's correlation coefficient (with an indication of strength) | ||

Are the means or medians the same? | Scalar | Two | Student's T-test | Mann-Whitney U-test | |

Are the means or medians the same? | Scalar | More than two | Analysis of Variance (ANOVA) | Kruskal-Wallis | |

Can I predict one from another? | Scalar | Two | More than two independent | Regression or multiple regression |

### One or two tails?Edit

When we formulate our hypothesis involving the comparison of values for a parameter or statistic, we choose whether to ask the question in one of two ways. We might simply ask *are the values different* or we might ask *is one value smaller (or greater) than the other*. In the first case we will determine the outcome using a *two tailed test* and in the second case, using a *one tailed test*.

## NotesEdit

- ↑ This is not true: it is possible to test the assocation of more than two nominal variables but the design is complicated