Statistical Analysis Biology

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  • Created on: 16-09-13 04:24
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1.1 Statistical Analysis
1.1.1 State that error bars are a graphical representation of the variability of
Bars on a graph extending above and below the mean value showing the
variability of the data.
Can represent the range of data/standard deviation
Larger error bars: less reliable data
Smaller error bars: more reliable data
1.1.2 Calculate the mean and standard deviation of a set of values
Three types of average:
Median: the middle value of a range of results
Mode: the value that appears the greatest nos. of times
Mean: the sum of all the results divided by the nos. of results
If the mean, mode and median are approximately the same we can usually
assume that the data shows a `normal distribution': Bell shaped frequency
Standard deviation:
Calculator: Menu (2:Stat), List, F2, F1(1VAR), sx(standard deviation)
1.1.3 State that the term standard deviation is used to summarize the spread of
values around the mean and that 68% of the values fall within one standard
deviation of the mean
1.1.4 Explain how standard deviation is useful for comparing the means and the
spread of data between two or more samples.
Small standard deviation: data is clustered around the mean value, a more
reliable mean value
Large standard deviation: Date is more spread out, mean is less reliable
Can be used to say whether a particular result lies outside the normal range
of data
Can't be used to compare two different samples (ttest)
1.1.5 Deduce the significance of the difference between two sets of data using
calculated values for t and appropriate tables
Any significant difference between two data sets
Calculate a "pvalue": The probability that there is no difference between the
two samples
High P values: Small differences (high probability there is no significant
differences between results)
P>0.05: There is no significant difference between the two samples
P<0.05: There is a significant difference between the results
Converting a ttest result (tvalue) to a pvalue:
"Table of tValues" Pg. 7 of textbook
1. Work out the nos of degrees of freedom (DF)
o DF=total nos. of results ­ 2
o E.g. Sample 1 (9,5,4,3,2) Sample 2 (6,9,12,14,5): DF=102=8

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Go down the DF column in the table of tvalues until you reach your DF
o E.g. 8
3. Read across until you get to the closest value that is smaller than your
calculated t value
4. Read down to the bottom row which gives you "p"
1.1.6 Explain that the existence of a correlation does not establish that there is
a causal relationship between two variables.…read more


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