# Module 3: Data, Evidence, and Decision Making

## Patterns and Trends

### Data vs Evidence

- The terms data and evidence are often used interchangeably, but in scientific inquiries the terms refer to two different concepts.
- Data is essentially pure information.
- The results of a scientific inquiry (e.g. a table or spreadsheet) with no interpretation is an example of data.
- There is no context or information attached.

- Evidence is data with context.
- While data can exist independently, when conclusions and analyses are made from it, it is evidence.
*The data becomes evidence for a statement.*- Data is only evidence when there is an opinion, viewpoint, or argument that it reinforces or refutes.

- Data has no meaning alone, it must be in the form of evidence to be of any use.

## Statistics in Scientific Research

### Mean, Median, and Standard Deviation

- Mean refers to the average of a dataset
- Mean is calculated using $\frac{\text{Sum of values}}{\text{Number of values}},$ and is represented by either $\mu$ or $\bar{x}\newcommand{orange}{\color{orange}}$
- Median refers to the middle value of the dataset, and is the term at position $\frac{n}{2}$ where $n$ is the number of values in the dataset
- Median is represented by $Med(X),$ where $X$ is the relevant variable
- Standard deviation is the amount of variation in a dataset
- Standard Deviation can be calculated using $\orange{\sqrt{\frac{\sum{(x_{i}-\mu)^{2}}}{n}}}$ where n is the number of values in the dataset, and $x_i$ represents each value of the variable
- Standard Deviation is represented by $\sigma$, and is the square root of variance

## Statistics Tests

### F-Test

#### When is it used?

- When you have 2 numerical datasets, and want to compare their variances (how much they deviate from their respective means)

#### What does it tell us?

- The further the result of the F-test is from 1, the stronger the evidence for unequal population variances.
- Therefore, higher F-statistics can be interpreted as less correlation between the two variables.

#### How is an F-statistic calculated?

Define the null hypothesis $(H_0)$ as “The two variables have equal variance,” and the alternate hypothesis $(H_1)$ as “the two variables have unequal variances.”

- Don’t actually write that if you’re asked. Instead, write “
*variable 1*is dependent on*variable 2*.” for the null, and “*variable 1*is not dependent on*variable 2*.”

- Don’t actually write that if you’re asked. Instead, write “
Calculate the statistic using $\orange{F=\frac{\sigma_1}{\sigma_2}}$ where $\sigma_1$ is the is the larger variance.

#### How can a conclusion be drawn from the F-test?

- Use an f-statistic table to determine the critical F-value of the dataset:
- The significance/alpha level will be written above the table, usually in the form of $\alpha=0.05$ (where 0.05, or 5%, is the significance level). If the question doesn’t give you the significance level, assume 0.05.
- The numerator’s degrees of freedom (number of values of variable 1, minus 1) is along the top
- The denominator’s degrees of freedom (number of values of variable 2, minus 1) is along the left side
- You’ll be given the table for any in-class test. There’s also one here.

- If the calculated F-statistic is lower than the critical F value, accept the null hypothesis. If the calculated f-statistic is greater than the critical value, reject the null hypothesis.

### T-Test

#### When is it used?

- To compare 2 normally-distributed variables with unknown variances
- Can be used alongside the F-Test

#### What does it tell us?

- T-test determines whether there is a significant difference between th means of 2 groups of data.
- Results from a T-test can be used as evidence for correlation between 2 variables.

#### How is the T-Statistic calculated?

- Identify the mean $(\bar{x}),$ standard deviation $(\sigma),$ and number of values $(n)$ for each group.
- Establish a null hypothesis stating that mean 1 $(\bar{x}_1)$ and mean 2 $(\bar{x}_2)$ are equal.
- This will usually be phrased as
*$H_0:$ There is no difference between variables 1 and 2.* - Following this with an alternate hypothesis is also a good idea.

- This will usually be phrased as
- Use the formula for T-test with 2 variables:

$$\orange{T=\frac{\bar{x}_1-\bar{x}_2}{\sqrt{\frac{{\sigma_1}^2}{n_1}+\frac{{\sigma2}^2}{n_2}}}}$$

If you only have 1 variable, use the 1-var t-test:

$$\orange{T=\frac{\bar{x}-H_0}{\frac{\sigma}{\sqrt{n}}}}$$

#### How is the T-statistic interpreted?

- Usually, a p-value is used to interpret the t-statistic. However, critical T-values can also be found using yet another table.
- This time, a few extra steps are needed:

- Determine your significance/alpha level (assume $\alpha=0.05$) unless told otherwise.
- Determine if your test is 1-tailed or 2-tailed:
- Rephrase the question as an equation (for example, from “25% of packets are too heavy” to “Too heavy > 25%")
- If the equation has “greater than” or “less than”, you need a 1-tailed t-test
- If the equation has “equals”, you need a 2-tailed t-test

- Now we move to the Empirical rule: because your data is normally distributed, you need to determine how many standard deviations from the mean your data can fall.
- For example, an alpha value of 0.05 (or 5%) means data needs to fall between $\bar{x}-2\sigma$ and $\bar{x}+2\sigma$ (because 95% of values are $-2\leq z\leq2$).
- If your test is 2-tailed, half your alpha level (because you’re only looking at 1 side of a symmetrical distribution)

- Use a t-score table to determine the critical t-value. If your calculated T-value is greater than the critical value,

#### How can conclusions be drawn?

- If the calculated t-value is greater than the critical value, reject the null hypothesis. Otherwise, accept the null hypothesis.

#### I still don’t get it :/

Crash Course Statistics has a good video on T-tests that explains it far better than I have here.

### Chi-Squared Test $(\chi^2)$

#### When is it used?

- To determine whether a categorical variable fits an expected distribution.
- Can only be used for discrete variables, such as frequency.

#### What does it tell us?

- Chi-Squared determines whether the difference between the observed and theoretical distributions is significant enough to be meaningful.
- Variation between observed and expected might be chanc/randomness, but a Chi-Squared test will determine the likelihood of this.

#### How is Chi-Squared calculated?

$$\orange{\chi^{2}=\frac{(O_1-E_1)^{2}}{E_1}+\frac{(O_2-E_2)^{2}}{E_2}+…}$$

Where $O$ is the observed value, and $E$ is the expected value.

#### How can $\chi^2$ be interpreted?

- Identify degrees of freedom: (number of rows minus 1) times (number of columns minus 1)
- Identify alpha/significance level (usually $\alpha=0.05$)
- Use a chi-squared table to determine the critical value.
- If the calculated chi-squared value is greater than the critical value, reject the null hypothesis. Otherwise, accept it.

#### Crash course?

### Analysis of Variance (ANoVA)

#### When is it used?

- Analysis of Variance is used to analyse variance (🙄)
- It compares the amount of variance
*within*each group to the variance*between*groups.

#### What does it tell us?

- If variance within each group is high, but between groups is low, it’s likely caused by an external influence.
- If variance within groups is low but between groups is high, it’s likely that the property being measured is dependent on the group the sample was taken from.

#### How is ANoVA calculated?

- Calculate the mean of each group $(x_g,\text{ where g is the group number})$, as well as the mean of all the groups combined $(\bar{x})$
- Calculate the SSR (sum of squares regression) using the formula $SSR=n\left[\left(x_1-\bar{x}^2\right)+\left(x_2-\bar{x}^2\right)+…\right]$ where n is the sample size of all groups combines, and $x_1,x_2,…$ are the means of their respective groups.
- Calculate the SSE (sum of squares error) using the formula $SSE=\sum{(x_{ij}-x_i)^2}$ where $x_{ij}$ is value j in group i, and $x_i$ is the mean of group i
- Calculate the SST (sum of squares total) as SSR+SSE

#### How is ANoVA interpreted?

- Conveniently, we can use the same method as we did for the F-test (even the same distribution tables)
- The significance/alpha level will be written above the table, usually in the form of $\alpha=0.05$ (where 0.05, or 5%, is the significance level). If the question doesn’t give you the significance level, assume 0.05.
- The numerator’s degrees of freedom (number of groups, minus 1) is along the top
- The numerator’s degrees of freedom (number of values across all groups, minus 1) is along the left side
- You’ll be given the table for any in-class test. There’s also one here.
- If the calculated ANoVA is lower than the critical value, accept the null hypothesis. If the calculated ANoVA is greater than the critical value, reject the null hypothesis.

#### Crash Course?

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