Statistics Flashcards
(28 cards)
Position of mean median and mode in the normal distribution
The mean median and mode are the SAME in a normal distribution
Effect of the symmetricality of the normal distribution
50% of the population lie above the mean and 50% lie below the mean
What is the range of a set of data
Range = largest value – smallest value
The range tells us what the total spread of the data is.
Problems with using the range to look at the spread of a data set
The range uses the highest and lowest data points which may be extreme (outliers) therefore giving a false impression of how spread the data really is.
Define standard deviation and what it measures
A measure of the spread of the data
This is a measure of the spread of data on either side of the mean. It is a measure of variability.
is standard deviation as affected by outliers as the range
Standard deviation (SD) uses all the data and is less affected by outliers.
However The standard deviation is only meaningful provided there are sufficient data points (usually a minimum of 5).
What is the purpose of a statistical test
To determine the probability that your results could have occurred by chance.
(and not due to a biological causal mechanism)
A statistical test enables us to calculate the probability that our results are real or due to chance.
How statistically certain must you be of your results
It is generally accepted that we want to be at least 95% certain that our results are real (or 5% certain that our observed results were due to chance).
This can be expressed as a probability (p = 0.05).
What is spearman’s rank correlation coefficient
Spearman’s Rank Correlation is a statistical test to test whether there is a significant relationship between two sets of data.
The Spearman’s Rank Correlation test can only be used if there are at least 10 (ideally at least 15) pairs of data.
Define correlation
Where a change in one variable is associated with a change in another variable.
What does a strong spearman’s rank indicate
Causation implies that one variable causes the other variable to happen i.e. there is an underlying causal mechanism.
Interpreting the values for spearman’s correlation coefficient
rs will be a number between – 1 and +1
A negative value for rs implies a negative correlation
A positive value for rs implies a positive correlation
What are degrees of freedom
the number of values in a data set that are free to vary.
How to calculate the degrees of freedom
Degrees of freedom = n - 2
What does a p value of 0.05 mean
spearmans
p = 0.05 means that there is a 5% probability that our correlation is due to chance alone.
Alternatively we can say that there is a 95% probability that any correlation is not due to chance i.e. there is an established biological causal factor or mechanism.
What does a T test do
Compares means
What does a T test require
Data that a mean can be calculated from
What does a T test measure
The significance of the difference between observed data and expected results when data fits into categories
Compares the means of set sets of data
Unpaired vs paired T test
Paired t test compares data from the same group before and after a change
Unpaired T test tests two separate/ independent sets of data
How to make a study more valid
increase sample size
Use similar group sizes
Ensure similar starting conditions- e.g. health conditions or gender
T test assumptions
Continuous variables
Variables are normally distributed
Roughly equal sample sizes
Each group has had unbiased samples taken
How to calculate degrees of freedom for T test
(n1 - 1) + (n2 -1)
What does a t value that this larger than the critical value mean for a conclusion
reject null hypothesis
95% certainty that there is a significant difference between the two means with an underlying causal relationship
What is chi squared used for
Used to compare observed results with expected results.
The data must be categoric e.g. number of red flowers, white flowers
