Transcript STATISTICS!!!

Statistics in Science
Data
can be collected about a
population (surveys)
Data
can be collected about a
process (experimentation)
STATISTICS!!!
The science of data
2 types of Data
Qualitative
Quantitative
Qualitative Data
Information that relates to
characteristics or description
(observable qualities)
 Information is often grouped by a
descriptive category
 Examples

– Species of plant
– Type of insect
– Shades of color
– Rank of flavor in taste testing
Remember: qualitative data can be “scored” and
evaluated numerically
Qualitative data, manipulated
numerically

Survey results, teens and need for environmental action
Quantitative data
 Quantitative
– measured using
a naturally occurring
numerical scale
 Examples
–Chemical concentration
–Temperature
–Length
–Weight…etc.
Quantitation
 Measurements
graphically
are often displayed
Quantitation = Measurement

In data collection for Biology, data must
be measured carefully, using laboratory
equipment
(ex. Timers, metersticks, pH meters, balances , pipettes, etc)
 The limits of the equipment used add
some uncertainty to the data collected. All
equipment has a certain magnitude of
uncertainty. For example, is a ruler that is
mass-produced a good measure of 1 cm?
1mm? 0.1mm?

For quantitative testing, you must
indicate the level of uncertainty of the
tool that you are using for
measurement!!
How to determine uncertainty?
Usually the instrument manufacturer will
indicate this – read what is provided by
the manufacturer.
 Be sure that the number of significant
digits in the data table/graph reflects the
precision of the instrument used (for ex. If
the manufacturer states that the accuracy
of a balance is to 0.1g – and your average
mass is 2.06g, be sure to round the
average to 2.1g) Your data must be
consistent with your measurement tool
regarding significant figures.

 Any
lab you design for AP/IB Biology
must have both quantitative and
qualitative data
Quick Review – 3 measures of
“Central Tendency”
Quantitative data
 mean: sum of data points divided by
the number of points
Quantitative or qualitative data
 mode: value that appears most
frequently
 median: When all data are listed from
least to greatest, the value at which
half of the observations are greater,
and half are lesser.
Comparing Means
 Once
the means are calculated
for each set of data, the average
values can be plotted together
on a graph, to visualize the
relationship between each set of
data.
The Average Rate of Growth
On Various Types of Trees
Growth in meters
16
12
8
4
0
beech
maple
hickory
Type of Trees Measured
oak
Error Bars
 Are
a graphical representation of the
variability of data.
Drawing error bars
 The
simplest way to draw an error
bar is to use the mean as the central
point, and to use the distance of the
measurement that is furthest from
the average as the endpoints of the
data bar
Value farthest
from average
Calculated
distance
Average
value
The Average Rate of Growth
On Various Types of Trees
Growth in meters
16
12
8
4
0
beech
maple
hickory
Type of Trees Measured
oak
What do error bars suggest?
 If
the bars show extensive overlap, it
is likely that there is not a significant
difference between those values
Error bars present
evidence so readers can
verify that the authors'
reasoning is correct.
How can leaf lengths be displayed
graphically?
Simply measure the lengths of each and plot how
many are of each length
If smoothed, the histogram data
assumes this shape
This Shape?


Is a classic bell-shaped curve, AKA
Gaussian Distribution Curve, AKA a
Normal Distribution curve.
Essentially it means that in all studies with
an adequate number of data points (>30)
a significant number of results tend to be
near the mean. Fewer results are found
farther from the mean
 The
standard deviation is a
statistic that tells you how tightly all
the various examples are clustered
around the mean in a set of data
Standard deviation
 The
STANDARD DEVIATION is a
more sophisticated indicator of the
precision of a set of a given number
of measurements
– The standard deviation is like an
average deviation of measurement
values from the mean. The standard
deviation can be used to draw error
bars, instead of the maximum deviation.
A typical standard distribution curve
According to this curve:
 One
standard deviation away from
the mean in either direction on the
horizontal axis (the red area on the
preceding graph) accounts for
somewhere around 68 percent of the
data in this group.
 Two standard deviations away from
the mean (the red and green areas)
account for roughly 95 percent of the
data.
Three Standard Deviations?
 three
standard deviations (the red,
green and blue areas) account for
about 99 percent of the data
-3sd -2sd
+/-1sd
2sd
+3sd
How is Standard Deviation
calculated?
With this formula!
AGHHH!
DO
I NEED TO
KNOW THIS
FOR THE
TEST?????
Not the formula!


This can be calculated on a scientific calculator
OR…. In Microsoft Excel, type the following code
into the cell where you want the Standard
Deviation result, using the "unbiased," or "n-1"
method: =STDEV(A1:A30) (substitute the cell
name of the first value in your dataset for A1,
and the cell name of the last value for A30.)
You DO need to know the concept
& use it in your lab reports!
Standard deviation is a statistic that
tells how tightly all the various data
points are clustered around the mean in a
set of data.
 When the data points are tightly bunched
together and the bell-shaped curve is
steep, the standard deviation is
small.(precise results, smaller sd)
 When the data points are spread apart
and the bell curve is relatively flat, a large
standard deviation value suggests less
precise results

Usefulness of SD
Look at the data given for bean plants
Height of bean plants in the
sunlight in cm (+0.01 cm)
Height of bean plants in the
shade in cm (+0.01 cm)
124
131
120
60
153
160
98
212
123
117
142
65
156
155
128
160
139
145
117
95
Total 1300
SD: 17.68 cm
Total
1300
SD: 47.02 cm
What is the mean for each
sample?
Both are 130 cm
Now look at the variations of
each sample.
The plants in the shade
are more variable than the
ones in the sunlight. What
does this suggest?
Other factors may be
influencing the growth in
addition to sunlight and shade.
SD allows you to mathematically
quantify the variation observed.
 The
high SD of the bean plants in the
shade indicates a very wide spread
of data around the mean.
– This should make you question the
experimental design.
 EX:
The plants in the shade are growing in
different soil types.
 So…don’t
just look at the means;
they don’t offer the full picture 
Try this question…
 The
lengths of a sample of tiger
canines were measured. 68% of the
lengths fell within a range between
15 mm and 45 mm. The mean was
30 mm. What is the standard
deviation of this sample?
15mm
Let’s do this…
The t-test
Used to determine whether or not the
difference between 2 sets of data is a
significant (real) difference.
 Used to test the statistical significance
between the means of two samples
 When given the calculated value of t, you can
use a table of t values (handout).
 On the left hand column is “Degrees of
Freedom”.

– This is the sum of sample sizes of each group
minus 2.
If the degrees of freedom is 9, & if the given value
of t is 2.60, the table indicates that the t value is
greater than 2.26.
 WHAT
DOES THIS MEAN???
When you look at the bottom of the table,
you will see that the probability that
chance alone could produce the result is
only 5% (0.05).
 This means that there is a 95% chance
that the difference is significant.

SO…
Large t-values mean little overlap between
two sets of data; difference between them
 Small t-values mean much overlap and
probably no difference
 Calculated t<critical t value = differences
between data are not significant = null
hypothesis not rejected
 Calculated t>critical t value = differences
are significant = null hypothesis
rejected.

Compare 2 groups of barnacles living
on a rocky shore.



You are measuring the width of their shells to see if a
significant size difference is found depending on how close
they live to the water.
– One group lives 0-10 meters from water
– The other group lives 10-20 meters.
– 15 shells from each group were measured.
The mean of the group closer to the water indicated that
living closer to the water causes the barnacles to have a
larger shell.
If the value of t is 2.25, is that a significant difference?
The degree of freedom is 28. So the p =0.05, which means the
probability that chance alone could produce this result is 5%.
The confidence level is 95%. So, barnacles living nearer the water have a
significantly larger shell than those living 10meters or more away from
the water.
CORRELATION
AND
CAUSATION
EX: Africanized Honey Bees
(AHBs)
These bees have migrated to the
southwestern states of the US.
 They have not migrated to the
southeastern states.
 The edge of the areas where AHBs are
found coincides with the point where there
is an annual rainfall of 55inches.
 This seems to be a barrier to the
migration of the bees.

This is an example of a mathematical correlation & is not evidence of a
cause.
Correlation and cause
 Observations
without
experimentation show correlation
 Experimentation is necessary to
show cause
Using A Mathematical
Correlation Test
r value is the correlation
 Value of r can vary:
– r=1 means completely positive
correlation
– r=-1 means completely negative
correlation
– r=0 means no correlation
Say we were trying to
determine, among cormorant
birds, if there is a correlation
between the sizes of males &
females which breed together.



Data is collected and an r value of 0.88 is
determined.
What does this mean?
It shows a positive correlation between the
sizes of the 2 sexes.
– In other words, large females mate with large males.
Remember Correlation is not
Causation
 How
would cause be determined?