Finding & Using Standard Deviation

Entry Task

What trends do you see in your experimental results?

How confident are you in your data? (very confident, not very confident)

Why?

Do you think you can make a strong conclusion from your results?

Have Out

Speed Bumps completed

Bar Graphs

Reading Summaries

Finding Standard Deviation

1. Find the mean (or average) of your samples

2. Find the variance① Find the difference from the mean for each

sample number

② Square each difference from the mean

③ Find the average squared difference from the mean

3. The Standard Deviation is the square root of the variance

Practice Calculating Standard Deviation

15 minutes

Calculate SD for your experiment

(10 minutes)

Check your results with your group. You should all find the same SD for your data.

Why Standard Deviation?

Standard deviation tells you how spread out the results for a condition are

• Are they concentrated around an area?

• Are they spread far and wide?

• Mean seems to show stronger Product Reliability…. Until you look at SD

• The large SD shows respondents were polarized and product reliability varied for consumers

Why Standard Deviation?

If our data are spread out (large SD), Our results may not be

reliable.

This usually happens when an important variable is not controlled well or maybe

not at all

OR we haven’t collected enough data

Graph your standard deviation onto your bar graph

To Graph the SD,

1. Add your SD to your average. You should have a number larger than the average for that condition.

2. Find that point on your graph above the bar for that condition

3. Draw a line from the top of the bar to the point

4. Subtract your SD from your average. You should have a number smaller than your average for that condition.

5. Repeat steps 2 & 3 but below the top of the bar

Interpreting Standard Deviation

SD bars tell us the range where our data are clustered.

When SD bars for two conditions overlap, that means the overlapping portion could have been produced by either condition.

If we are expecting to prove a difference between two conditions, the more the bars overlap, the lower our confidence that there is a difference between the conditions.

Practice Interpreting Standard Deviation

What do your standard deviation bars seem to tell you about your experiment?

15 minutes

Homework:

1. Finish! Science Fair Assignment #1 DUE 10/01/15

2. Write a conclusion for your Speed Bumps lab

• including information from your SD calculations / bars to explain the strength of your confidence in your data/ conclusion.