# Statistics - Hypothesis Testing a Mean (Left Tailed)

A population mean is an average of value a population.

Hypothesis tests are used to check a claim about the size of that population mean.

## Hypothesis Testing a Mean

The following steps are used for a hypothesis test:

- Check the conditions
- Define the claims
- Decide the significance level
- Calculate the test statistic
- Conclusion

For example:

**Population**: Nobel Prize winners**Category**: Age when they received the prize.

And we want to check the claim:

```
```"The average age of Nobel Prize winners when they received the prize is **less** than 60"

By taking a sample of 30 randomly selected Nobel Prize winners we could find that:

```
```The mean age in the sample (\(\bar{x}\)) is 62.1

The standard deviation of age in the sample (\(s\)) is 13.46

From this sample data we check the claim with the steps below.

## 1. Checking the Conditions

The conditions for calculating a confidence interval for a proportion are:

- The sample is randomly selected
- And either:
- The population data is normally distributed
- Sample size is large enough

A moderately large sample size, like 30, is typically large enough.

In the example, the sample size was 30 and it was randomly selected, so the conditions are fulfilled.

**Note:** Checking if the data is normally distributed can be done with specialized statistical tests.

## 2. Defining the Claims

We need to define a **null hypothesis** (\(H_{0}\)) and an **alternative hypothesis** (\(H_{1}\)) based on the claim we are checking.

The claim was:

```
```"The average age of Nobel Prize winners when they received the prize is **less** than 60"

In this case, the **parameter** is the mean age of Nobel Prize winners when they received the prize (\(\mu\)).

The null and alternative hypothesis are then:

```
```**Null hypothesis**: The average age was 60.

**Alternative hypothesis**: The average age was **less** than 60.

Which can be expressed with symbols as:

```
```\(H_{0}\): \(\mu = 60 \)

\(H_{1}\): \(\mu < 60 \)

This is a '**left** tailed' test, because the alternative hypothesis claims that the proportion is **less** than in the null hypothesis.

If the data supports the alternative hypothesis, we **reject** the null hypothesis and **accept** the alternative hypothesis.

## 3. Deciding the Significance Level

The significance level (\(\alpha\)) is the **uncertainty** we accept when rejecting the null hypothesis in a hypothesis test.

The significance level is a percentage probability of accidentally making the wrong conclusion.

Typical significance levels are:

- \(\alpha = 0.1\) (10%)
- \(\alpha = 0.05\) (5%)
- \(\alpha = 0.01\) (1%)

A lower significance level means that the evidence in the data needs to be stronger to reject the null hypothesis.

There is no "correct" significance level - it only states the uncertainty of the conclusion.

**Note:** A 5% significance level means that when we reject a null hypothesis:

We expect to reject a **true** null hypothesis 5 out of 100 times.

## 4. Calculating the Test Statistic

The test statistic is used to decide the outcome of the hypothesis test.

The test statistic is a standardized value calculated from the sample.

The formula for the test statistic (TS) of a population mean is:

\(\displaystyle \frac{\bar{x} - \mu}{s} \cdot \sqrt{n} \)

\(\bar{x}-\mu\) is the **difference** between the **sample** mean (\(\bar{x}\)) and the claimed **population** mean (\(\mu\)).

\(s\) is the sample standard deviation.

\(n\) is the sample size.

In our example:

```
```The claimed (\(H_{0}\)) population mean (\(\mu\)) was \( 60 \)

The sample mean (\(\bar{x}\)) was \(62.1\)

The sample standard deviation (\(s\)) was \(13.46\)

The sample size (\(n\)) was \(30\)

So the test statistic (TS) is then:

\(\displaystyle \frac{62.1-60}{13.46} \cdot \sqrt{30} = \frac{2.1}{13.46} \cdot \sqrt{30} \approx 0.156 \cdot 5.477 = \underline{0.855}\)

You can also calculate the test statistic using programming language functions:

### Example

With Python use the scipy and math libraries to calculate the test statistic.

```
import scipy.stats as stats
```

import math

# Specify the sample mean (x_bar), the sample standard deviation (s), the mean claimed in the null-hypothesis (mu_null), and the sample size (n)

x_bar = 62.1

s = 13.46

mu_null = 60

n = 30

# Calculate and print the test statistic

print((x_bar - mu_null)/(s/math.sqrt(n)))

Try it Yourself »
### Example

With R use built-in math and statistics functions to calculate the test statistic.

```
# Specify the sample mean (x_bar), the sample standard deviation (s), the mean claimed in the null-hypothesis (mu_null), and the sample size (n)
```

x_bar <- 62.1

s <- 13.46

mu_null <- 60

n <- 30

# Output the test statistic

(x_bar - mu_null)/(s/sqrt(n))

Try it Yourself »
## 5. Concluding

There are two main approaches for making the conclusion of a hypothesis test:

- The
**critical value**approach compares the test statistic with the critical value of the significance level. - The
**P-value**approach compares the P-value of the test statistic and with the significance level.

**Note:** The two approaches are only different in how they present the conclusion.

### The Critical Value Approach

For the critical value approach we need to find the **critical value** (CV) of the significance level (\(\alpha\)).

For a population mean test, the critical value (CV) is a **T-value** from a student's t-distribution.

This critical T-value (CV) defines the **rejection region** for the test.

The rejection region is an area of probability in the tails of the standard normal distribution.

Because the claim is that the population mean is **less** than 60, the rejection region is in the left tail:

The size of the rejection region is decided by the significance level (\(\alpha\)).

The student's t-distribution is adjusted for the uncertainty from smaller samples.

This adjustment is called degrees of freedom (df), which is the sample size \((n) - 1\)

In this case the degrees of freedom (df) is: \(30 - 1 = \underline{29} \)

Choosing a significance level (\(\alpha\)) of 0.05, or 5%, we can find the critical T-value from a T-table, or with a programming language function:

### Example

With Python use the Scipy Stats library `t.ppf()`

function find the T-Value for an \(\alpha\) = 0.05 at 29 degrees of freedom (df).

```
import scipy.stats as stats
```

print(stats.t.ppf(0.05, 29))

Try it Yourself »
### Example

With R use the built-in `qt()`

function to find the t-value for an \(\alpha\) = 0.05 at 29 degrees of freedom (df).

```
qt(0.05, 29)
```

Try it Yourself »
Using either method we can find that the critical T-Value is \(\approx \underline{-1.699}\)

For a **left** tailed test we need to check if the test statistic (TS) is **smaller** than the critical value (CV).

If the test statistic is smaller the critical value, the test statistic is in the **rejection region**.

When the test statistic is in the rejection region, we **reject** the null hypothesis (\(H_{0}\)).

Here, the test statistic (TS) was \(\approx \underline{0.855}\) and the critical value was \(\approx \underline{-1.699}\)

Here is an illustration of this test in a graph:

Since the test statistic was **bigger** than the critical value we **keep** the null hypothesis.

This means that the sample data does not support the alternative hypothesis.

And we can summarize the conclusion stating:

```
```The sample data does **not** support the claim that "The average age of Nobel Prize winners when they received the prize is less than 60" at a **5% significance level**.

### The P-Value Approach

For the P-value approach we need to find the **P-value** of the test statistic (TS).

If the P-value is **smaller** than the significance level (\(\alpha\)), we **reject** the null hypothesis (\(H_{0}\)).

The test statistic was found to be \( \approx \underline{0.855} \)

For a population proportion test, the test statistic is a T-Value from a student's t-distribution.

Because this is a **left** tailed test, we need to find the P-value of a t-value **smaller** than 0.855.

The student's t-distribution is adjusted according to degrees of freedom (df), which is the sample size \((30) - 1 = \underline{29}\)

We can find the P-value using a T-table, or with a programming language function:

### Example

With Python use the Scipy Stats library `t.cdf()`

function find the P-value of a T-value smaller than 0.855 at 29 degrees of freedom (df):

```
import scipy.stats as stats
```

print(stats.t.cdf(0.855, 29))

Try it Yourself »
### Example

With R use the built-in `pt()`

function find the P-value of a T-Value smaller than 0.855 at 29 degrees of freedom (df):

```
pt(0.855, 29)
```

Try it Yourself »
Using either method we can find that the P-value is \(\approx \underline{0.800}\)

This tells us that the significance level (\(\alpha\)) would need to be smaller 0.80, or 80%, to **reject** the null hypothesis.

Here is an illustration of this test in a graph:

This P-value is far **bigger** than any of the common significance levels (10%, 5%, 1%).

So the null hypothesis is **kept** at all of these significance levels.

And we can summarize the conclusion stating:

```
```The sample data does **not** support the claim that "The average age of Nobel Prize winners when they received the prize is less than 60" at a **10%, 5%, or 1% significance level**.

## Calculating a P-Value for a Hypothesis Test with Programming

Many programming languages can calculate the P-value to decide outcome of a hypothesis test.

Using software and programming to calculate statistics is more common for bigger sets of data, as calculating manually becomes difficult.

The P-value calculated here will tell us the **lowest possible significance level** where the null-hypothesis can be rejected.

### Example

With Python use the scipy and math libraries to calculate the P-value for a left tailed hypothesis test for a mean.

Here, the sample size is 30, the sample mean is 62.1, the sample standard deviation is 13.46, and the test is for a mean smaller 60.

```
import scipy.stats as stats
```

import math

# Specify the sample mean (x_bar), the sample standard deviation (s), the mean claimed in the null-hypothesis (mu_null), and the sample size (n)

x_bar = 62.1

s = 13.46

mu_null = 60

n = 30

# Calculate the test statistic

test_stat = (x_bar - mu_null)/(s/math.sqrt(n))

# Output the p-value of the test statistic (left tailed test)

print(stats.t.cdf(test_stat, n-1))

Try it Yourself »
### Example

With R use built-in math and statistics functions find the P-value for a left tailed hypothesis test for a mean.

Here, the sample size is 30, the sample mean is 62.1, the sample standard deviation is 13.46, and the test is for a mean smaller 60.

```
# Specify the sample mean (x_bar), the sample standard deviation (s), the mean claimed in the null-hypothesis (mu_null), and the sample size (n)
```

x_bar <- 62.1

s <- 13.46

mu_null <- 60

n <- 30

# Calculate the test statistic

test_stat = (x_bar - mu_null)/(s/sqrt(n))

# P-value the p-value of the test statistic (left tailed test)

pt(test_stat, n-1)

Try it Yourself »
## Left-Tailed and Two-Tailed Tests

This was an example of a **left** tailed test, where the alternative hypothesis claimed that parameter is **smaller** than the null hypothesis claim.

You can check out an equivalent step-by-step guide for other types here: