Download Statistics for IB Biology

Statistics for IB Biology
Syllabus Topic 1
1.1.1: Error Bars
• State that error bars are graphical
representations of the variability of data
– Graph Pad tutorial
– Excel instructions
Interpreting Statistics
is as important as collecting the data
http://davidmlane.com/ben/cartoons.html
1.1.2: Mean and Standard Deviation
• Calculate the mean and SD of a set of
values
– Mean or μ or x bar = ∑ x
n
– Standard Deviation or S
– For this course we will use the sample std. dev.,
NOT the population std. dev.
1.1.3: Spread of Values
• The term standard deviation is used to
summarize the spread of values around the
mean.
• Sixty-eight percent (68%) of the values fall
within one standard deviation of the mean.
– 1st SD contains 34% of the data points on each
side of mean (34+34=68)
– 2nd SD contains 13.5% of the data points on each
side of the mean
– 3rd SD contains the remaining 2.5% of the data of
each side of the mean
SD=1
Standard Deviation
SD=3
SD=2
Graphs from: http://www.childrensmercy.org/stats/definitions/stdev.htm
1.1.4:Comparing Means
• Explain how the standard deviation is useful for comparing the
means and the spread of data between two or more samples
– What can you conclude when standard error bars do overlap? No
surprises here. When SE bars overlap, you can be sure the difference between
the two means is not statistically significant .
– What can you conclude when standard error bars do not overlap? When
standard error (SE) bars do not overlap, you cannot be sure that the difference
between two means is statistically significant. T-test is commonly used to
compare these groups.
– What if you are comparing more than two groups? If two SE error bars
overlap, you can be sure that a post test comparing those two groups will find
no statistical significance. However if two SE error bars do not overlap, you
can't tell whether a post test will, or will not, find a statistically significant
difference. ANOVA is commonly used for comparing multiple groups.
from - http://www.graphpad.com/articles/errorbars.htm
Data Never Lies, it just needs the
proper explanation
http://davidmlane.com/ben/cartoons.html
1.1.5: T-tests
• Deduce the significance of the difference
between two sets of data using calculated
values for t and the appropriate tables.
– used for comparing two groups of data
(ANOVA for 3 or more)
– GraphPad tutorial