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Biology
 

 

 ICT IN BIOLOGY

HOW BIG IS A PLANT CELL? CONTINUED

Standard deviation
The standard deviation is a measure of the variation of the results. For data that is evenly distributed each side of the mean (a normal distribution) 68% of the data lies within one standard deviation of the mean. So the value for the standard deviation will be higher if the variation in the sample is larger.

 

You can work out the mean, the standard deviation and other statistics on your TI calculator as follows:

Use the key

Press
This gives you the STAT menu.

You will already be in the EDIT menu. Press 1
This opens a spread sheet

You can see if there is anything stored in the columns of the spread sheet. Check this and if there are data you need to clear it.

To clear the spread sheet

  

Press
You will be in the EDIT menu. Press 4
You will get “ClrList”
Press then

You have now cleared column L1
Repeat the process (, 4, , , etc….) to clear the other columns

To enter data

Press
You will be in the EDIT menu.
Select 1
This opens a spread sheet
type in the first number, then
second number then
etc
If you are comparing two sets of data do the same thing in column L2

To find out mean, median, standard deviation and more besides!

Press
Use the cursor to move across to the CALC menu

You will find a menu which permits you to calculate a large number of statistical parameters. Most of what we want is in 1:1-Var Stats.

The first on the list.
Press
1-Var Stats appears
Press then and
You will get:

x (the mean)
 Sx (the standard deviation of the sample)
 sx (the estimate of the standard deviation of the population)
 n (the number in the sample)

Scroll down for more!

Med (the median)

Plotting a histogram to show a frequency distribution

You will also find it useful to see the distribution of your data as a frequency histogram. This too can be carried out on the calculator.

Open and select Plot1
Switch it to On by pressing .
Under type: select the histogram type of presentation.

Under type: select the histogram type of presentation.
Scroll to Xlist: and select the list you want to plot. Press then then scroll down to the list of data. Press .
There is no Ylist because the histogram plots the frequency of the different data in the Xlist in regular groups called class intervals. These are set automatically by the calculator.
Press then and the histogram will appear.

To adjust your histogram press and enter the following information:

Xmin is the lower limit of the lowest class interval
 Xmax is the upper limit of the highest class interval
 Xscl is the width of the class intervals
 Ymin set zero
 Ymax needs to be higher than the highest frequency
 Yscl and Xres can be left as the default values

When these are set press and inspect the histogram. If you want to adjust the histogram further press again and change the values.

If you want to see if the histogram approaches a normal distribution you may test this using the normal probability graph. This is the last type of graph in the series.

Return to and Plot 1. Scroll to Type: Then scroll through the list to the last one and press .

Press then . The graph that appears now will be a scattergram of points. If they fall into a diagonal line, the distribution of the data follows a normal distribution.

Points for consideration

  • It is said that plant cells are 20 to 300 µm in diameter. What is this figure based upon?
     

  • What result do you get if you calculate the mean and standard deviation of
    (a) your first five cells
    (b) the first 10 cells.
    Compare with the mean and standard deviation of the full sample of 15 cells.
    What happens to these statistics as the sample size increases.
     

  • Were any of your measurements very different from the rest?
    What would you do in a case like this?
     

  • Try comparing the cells from the same tissue in an old leaf with those of a young leaf.

 

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updated on 20/10/07
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© Paul Billiet 2007