## Statistical analysis

Statistical Analysis is used to calculate mean and standard deviation, t-Test, and correlation between data sets. We do not cover this as a specific unit, however, the information will be incorporated into our curriculum as well your Internal Assessments. This information has been modified from the old IB Biology curriculum.

**Correlation and Causation**

One of the most common errors we find is the confusion between correlation and causation in science. In theory, these are easy to distinguish — an action or occurrence can cause another (such as smoking causes lung cancer), or it can correlate with another (such as smoking is correlated with alcoholism). If one action causes another, then they are most certainly correlated. But just because two things occur together does not mean that one caused the other, even if it seems to make sense.

Correlation describes the strength and direction of a linear relationship between two variables

Is positive (x = y) or negative (x = - y)

Causation describes the relationship between two variables, where one variable has a direct effect on another

Correlation does not automatically indicate causation – just because two variables change in relation to one another, does not mean they are linked

E.g. CO2 levels and crime have both risen, but CO2 levels don't cause crime

Correlation describes the strength and direction of a linear relationship between two variables

Is positive (x = y) or negative (x = - y)

Causation describes the relationship between two variables, where one variable has a direct effect on another

Correlation does not automatically indicate causation – just because two variables change in relation to one another, does not mean they are linked

E.g. CO2 levels and crime have both risen, but CO2 levels don't cause crime

**Mean**

The sum of all the data points divided by the number of data points.

Measure of central tendency for normally distributed data.

DO NOT calculate a mean from values that are already averages.

DO NOT calculate a mean when the measurement scale is not linear (i.e. pH units are not measured on a linear scale

Measure of central tendency for normally distributed data.

DO NOT calculate a mean from values that are already averages.

DO NOT calculate a mean when the measurement scale is not linear (i.e. pH units are not measured on a linear scale

**Standard Deviation**

Averages do not tell us everything about a sample. Samples can be very uniform with the data all bunched around the mean or they can be spread out a long way from the mean. The statistic that measures this spread is called the standard deviation. The wider the spread of scores, the larger the standard deviation. For data that has a normal distribution, 68% of the data lies within one standard deviation of the mean

**How to Calculate the Standard Deviation:**

- Calculate the mean (x̅) of a set of data
- Subtract the mean from each point of data to determine (x-x̅). You'll do this for each data point, so you'll have multiple (x-x̅).
- Square each of the resulting numbers to determine (x-x̅)^2. As in step 2, you'll do this for each data point, so you'll have multiple (x-x̅)^2.
- Add the values from the previous step together to get ∑(x-x̅)^2. Now you should be working with a single value.
- Calculate (n-1) by subtracting 1 from your sample size. Your sample size is the total number of data points you collected.
- Divide the answer from step 4 by the answer from step 5
- Calculate the square root of your previous answer to determine the standard deviation.
- Be sure your standard deviation has the same number of units as your raw data, so you may need to round your answer.
- The standard deviation should have the same unit as the raw data you collected. For example, SD = +/- 0.5 cm.

**Student t-Test**

The Student’s t-test is a statistical test that compares the mean and standard deviation of two samples to see if there is a significant difference between them. In an experiment, a t-test might be used to calculate whether or not differences seen between the control and each experimental group are a factor of the manipulated variable or simply the result of chance.

The T-test is a test of a statistical significant difference between two groups. A "significant difference" means that the results that are seen are most likely not due to chance or sampling error. In any experiment or observation that involves sampling from a population, there is always the possibility that an observed effect would have occurred due to sampling error alone. But if result is "significant," then the investigator may conclude that the observed effect actually reflects the characteristics of the population rather than just sampling error or chance.

In any significance test, there are two possible hypothesis:

The T-test is a test of a statistical significant difference between two groups. A "significant difference" means that the results that are seen are most likely not due to chance or sampling error. In any experiment or observation that involves sampling from a population, there is always the possibility that an observed effect would have occurred due to sampling error alone. But if result is "significant," then the investigator may conclude that the observed effect actually reflects the characteristics of the population rather than just sampling error or chance.

In any significance test, there are two possible hypothesis:

Null Hypothesis:"There is not a significant difference between the two groups; any observed differences may be due to chance and sampling error." |
Alternative Hypothesis:"There is a significant difference between the two groups; the observed differences are most likely not due to chance or sampling error." |

How to calculate T:

A p-value s the probability of concluding there is a significant difference between the groups result when the null hypothesis is true (meaning, the probability of making the WRONG conclusion). In biology, we use a standard “p-value” of 0.05. This means that five times out of a hundred you would find a statistically significant difference between the means even if there was none.

- Calculate the mean (X) of each sample
- Find the absolute value of the difference between the means
- Calculate the standard deviation for each sample
- Square the standard deviation for each sample
- Divide each squared standard deviations by the sample size of that group.
- Add these two values
- Take the square root of the number to find the "standard error of the difference.
- Divide the difference in the means (step 2) by the standard error of the difference (step 7). The answer is your "calculated T-value."
- Determine the degrees of freedom (df) for the test. In the t-test, the degrees of freedom is the sum of the sample sizes of both groups minus 2.
- Determine the “Critical T-value” in a table by triangulating your DF and the “p value” of 0.05.
- Draw your conclusion:

If your calculated t value is greater than the critical T-value from the table, you can conclude that the difference between the means for the two groups is significantly different. We reject the null hypothesis and conclude that the alternative hypothesis is correct.

If your calculated t value is lower than the critical T-value from the table, you can conclude that the difference between the means for the two groups is NOT significantly different. We accept the null hypothesis.

A p-value s the probability of concluding there is a significant difference between the groups result when the null hypothesis is true (meaning, the probability of making the WRONG conclusion). In biology, we use a standard “p-value” of 0.05. This means that five times out of a hundred you would find a statistically significant difference between the means even if there was none.

**Key Terms**

mean data set
correlation uncertainties |
standard deviation
significance cormorants chi-square |
error bars
significant number degree of freedom r value |
causation
correlation p value probability |
t-test
variable range standard error |

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**Class Materials:**

__Error Analysis__

__Significant Figures__

__Precision Measurements and Uncertainties__

Precision Lab

Precision Lab

__Topic 1 Statistics__(ppt)

__Biostatistics Practical Problems__

__Graphing In Edexcel__

Graphing in Edexcel Practiceproblems

Graphing in Edexcel Practice

__Standard Deviation__(ppt)

__Standard Deviation__(notes)

__Standard Deviation Practice problems__

__Hydroponics Standard Deviation Practice problems__

__t-Test__(ppt)

__t-Test__(notes)

__Correlation and Causation__(ppt)

__Correlation and Causation__(notes)

__Correlation reading__

__Correlations of cancer__(pdf)

__Data set #1__(pdf)

__Data set #2__(pdf)

__Data set #3__(pdf)

__T-test reading__

__T-Testing in Biology__University of

__Statistics Review__

**Useful Links**

__Review of means__

Click

__here__for calculating SD with tools

Click

__here__for Flash Card questions on Statistical Analysis

Click

__here__for tips on Excel graphing.

“Using error bars in experimental Biology” by Geoff Cumming, Fiona Fidler, and David L. Vaux. (Journal of Cell Biology)

**Are two sets of data really different?**

__Click here to perform Student’s__

*t*-testClick

__here__to perform Student’s

*t*-test via copy and paste

__Example graph__(from The Biology Teacher, September 2013)

__Graphic Calculator Tour__

__Easy Calculation__

__Statistics calculator__

__MERLIN software for Excel__

__Chi-square calculator__

__Chi-square table__

__T-test calculator____Standard deviation reading__

T-Test Table, Excel and calculations can be found

__here.__

There are many statistical tools to establish a statistically significant correlation. read more

__here__or read an article about Cause and Correlation by Wisegeek

__here__.

__Difference Between Correlation and Causation article__

Excellent

__Handbook of Biological Statistics__from John MacDonald

__Basic Statistical Tools__, from the Natural Resources Management Department

And

__The Little Handbook of Statistical Practice__is very useful.

Sumanas

__statistics animations__

__Field Studies Council__stats page, including the t-test

__Open Door Website__stats page and help with graphs and tables.

__Making Population Pyramids on Excel__

__Spreadsheet Data Analysis Tutortial__

__Video over Table__

Making Table g

Making Table g

__Making Tables__

__This is an ecocolumn design you can use in the long-term IA’s 1__- from learner.org

__Here’s another ecocolumn design you can use for the long-term IA project__- from fastplants.org

**In The News:**

Ed Yong writes for Cancer Research UK on the

__WHO’s verdict on mobile phones and cancer.__Correlation vs cause!

__Epidemiology:__The Science of Cohort Studies. How do we generate lifetimes’ worth of data in studies in medicine? Ben Goldacre’s BBC Radio 4 documentary,

__Science: From Cradle to Grave.__An amazing discipline to work in, and one birth cohort study has been

__running for over 65 years!__

Click here for a funny article on

__the 9 circles of scientific hell.__

**Video Clips**

Watch Hans Rosling’s brilliant

__Joy of Statistics__here. For a short clip: