10.5 Quantitative investigation of variation Flashcards
What is interspecific variation
What is intraspecific variation
Variation between individuals of different species
Variation between individuals of the same species
What is sampling
Selecting a group of individuals to measure that will represent the whole target population
So if these individuals are representative of the whole population, the measurements can be relied upon
Give two reasons why samples are sometimes not representative
. Sampling bias: Eg how you select your sample is bias, as the investigator could choose individuals which support their conclusion deliberately or unwittingly.
. Chance: Even if sampling bias is avoided, there is still a chance that it is not representative, eg the selected daisies could all happen to be the tallest ones in the population
What is sampling bias
- How you select your sample is bias, as the investigator could choose individuals which support their conclusion deliberately or unwittingly.
Or eg if they are meant to get a group of daisies but some are in an area with mud and nettles, and some are in a clean area, if they take more from the clean area to measure, it is not representative
What is the best way to eliminate sampling bias
. Use random sampling:
Divide field into a grid of numbered lines by putting down two tape measures at right angles to each other
Then Random coordinate generator on computer
Take samples at each coordinate
What is the best way to eliminate chance from sampling process
. Use a larger sample size, so the more individuals that are selected, the smaller the probability that chance will influence result, and less influence anomalies will have
. Analysis of date collected by using statistic tests to find extent that chance influenced the data
What is continuous variation
What is discontinuous variation
. Characteristics like height or weight which change gradually
. Characteristics that have a limited number of possible values eg blood group, eye colour
What is a normal distribution curve
What 3 terms are associated with them
. It is a bell shape when there is continuous variation, eg height in humans
So the graph is symmetrical about a central value
Mean, mode, median
It is a normal distribution curve when these all have the same value
How do you calculate:
Mean
Median
Mode
Sum of values divided by the number of values
. Central or middle set of values, so you put them all in order from smallest to biggest and select the middle value
. The single value of a sample that occurs the most often
What is a skewed distribution curve
When the normal distribution curve is shifted to one side
Because the mean, mode and median all have different values
In a normal distribution curve, what does the mean represent
Advantage and problem with means
The mean represents the maximum height of the curve.
The mean gives an average value which is good for comparing one sample to another, however it doesn’t provide info about the range of values in the sample
What is the standard deviation of a normal distribution curve
What are the percentages that come with it
. It represents the width of the curve
So indicates the range of values on either side of the mean
Standard deviation is the distance from the mean to the point where curve changes from being convex to concave
(the point of inflection)
So from when it curves downwards to when it starts curving upwards
On the curve, 68% of all measurements lie within standard deviation
+ - 1.0 and 95% of measurements lie within +- 2.0
How do you calculate standard deviation
√Σ(x-x_)^2 / (n-1)
Σ is the sum of
x is measured value from sample
x_ is mean value
n is total number of values in sample
Example question:
x= 4,1,2,3,5,0
first find mean which is 2.5
Subtract 2.5 from each of the measured values, but since some of the numbers will be negative, you need to square them all (even the positive ones)
. Add all these squared numbers together = 17.5
. Divide this number by the original number of measurements subtract 1
which will be 17.5 / 5 = 3.5
Then square root it all to reverse the square you did earlier so √3.5
1.87
