Experimental design and analysis
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- Created by: amy.louise
- Created on: 14-01-16 20:46
What is probability?
How likely an event will happen
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How is probability expressed?
A number (P-value)
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What is the maximum and minimum value probability can be?
0 and 1
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What does the sum of probabilities for all outcomes equal to?
1
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How can you calculate probability?
number of times an event happens / number of repeats
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What does the null hypothesis state?
That there is no relationship between two measurements
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What does a small p-value suggest?
That there is strong evidence that the null hypothesis is wrong
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What does a large p-value suggest?
That there is strong evidence that the null hypothesis is correct
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What would a p-value above 0.05 suggest about the difference?
Its due to chance
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When are two events mutually exclusive?
When they can't occur at the same time
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What is conditional probability?
The probability of event A occurring if event B has occured
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What is the rule of subtraction?
Finding the probability of event A occurring by doing 1-(P A not occuring)
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What is the rule of multiplication?
Finding the probability of both events A and B occurring. (pA x pB if A has occured)
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What does statistical interference mean?
Drawing conclusions about an observation based on data
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What is random sampling?
Sampling when the probability of including an individual from a population is the same for all individuals and is independent of other samples
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What is bias sampling?
Sampling when the probability of an individual being picked to be sampled from a population is increased or decreased due to its characteristics
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What is non-independant sampling?
Sampling when the probability of an individual being picked from a population to be sampled is dependant on other individuals from the population
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What is a type I error?
When you reject a null hypothesis which is actually correct. You think theres statistical difference when there isn't
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When are type I errors most commonly made?
When there is multiple testing
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When would you use a bonferoni correction?
In order to reduce type I errors. When an experiment is testing many hypotheses
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What does the bonferoni correction do?
Tests hypotheses to a certain statistical significance. 0.05 / no. of hypotheses
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How does the bonferroni correction work?
Because the probability of getting at least one significant result increases as the number of hypotheses increases
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What is a type I error also known as?
A false negative
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What is a type II error?
When you accept the null hypothesis but its actually false. When you think theres no significant difference but there is
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How can you reduce the chance of making a type II error?
Ensuring your test has enough statistical power
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What is statistical error?
The likelihood that a study will detect an effect if there is a true difference to be seen
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What is statistical error mainly effected by?
Same size and size of the effect
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What is the effect size?
How big the effect of a variable had on samples
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Does binomial distribution had a certain number of repeats?
Yes
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What are the outcomes of a binomial experiment?
Success or failure
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What is Q in a binomial experiment?
The probability of failure (1-P)
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What is the binomial probability?
The probability that n-number of trials results in exactly X successes when the probability of a success is P
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Does binomial distribution had a certain number of repeats?
No it has infinite trials
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What has to be know in a poisson experiment?
The average number of successes that occurs within a specific region
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What can a 'region' be in a poisson distribution?
Time, length, volume
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What is the poisson probability?
The probability that exactly x successes will occur when there is a mean number of successes
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Wha type of data do you use a chi-squared test for?
Nominal data that falls into particular categories
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What does a one-factor chi-squared test do?
Compares expected values against observations of one variable
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What does a two-factor chi-squared test do?
It tests two variables and identifies whether the factors are independent of each other
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What is the null hypothesis in this test?
That there is no relationship
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What is continuous data?
Data that has a unit and can be measured
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What does the curve of normally distributed data depend on?
The mean and standard deviation from the mean
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What doe the mean determine on the graph?
The location of the centre of the graph
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What does the standard deviation determine?
The width and height of the graph
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What is the total area under the graph equal to?
1
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What % falls under 1 standard deviation of the mean?
68%
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What % falls under 2 standard deviation of the mean?
95%
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What % falls under 3 standard deviation of the mean?
99.7%
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What does the Shapiro-wilk test for?
Normality of data
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What does it allow you to see?
The distribution of your data
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hIf the P-value is below 0.05 is the null hypothesis accepted/ rejected?
rejected
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Is data normally distributed with a P-value of more than 0.05
Yes
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What is the t-test used for?
To estimate the population parameters when the sample size is small or the population variance is unknown
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What are parameters?
Numbers that summers data of a population. The mean and the standard deviation from the mean
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Can a population distribution be directly calculated?
No, only estimated
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What does the central limit theorem state?
That the sample mean of a statistic will follow a normal distribution as long as the sample size is large
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If your sample size is small or you don't know the standard deviation of your population, what can you do?
Look at the distribution of the t-statistic
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What does the standard error of the mean tell you?
It tells you how different the sample means are from the population mean
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What is a confident interval?
A range of estimated values that will likely contain the population mean
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What are confident limits?
The upper and lower boundaries of the confident interval
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Is the standard deviation of large samples always the same?
Yes at 1.96. over 30 samples
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What if you have a small sample size?
The standard deviations need to be adjusted
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Is the standard deviation of a small sample size bigger or smaller than that of a big sample size?
Bigger
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What is a t-value?
The difference between the standard deviation of the population and the sample.
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What is a confidence level?
A percentage that tells you how many of the samples you can expect to include the true population mean within their confident intervals
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What does a wide confidence interval suggest?
That you need a bigger sample size?
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Is it good to have a narrow confidence interval and why?
Yes because it is more precise
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How do you calculate variance?
Squared standard deviation
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How do you calculate the confidence interval?
The sample mean + and - the standard error
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What does a two-sample t-test measure?
It measures if themes of the two samples differer from the population mean and from each other
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What is the test statistic of this test?
Z
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When do you use ANOVA?
When you are comparing two or more variables
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What are the assumptions of ANOVA?
The variance of all the errors are the same, The errors are independent and they are normally distributed
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What does ANOVA test?
If the value of the variables differers significantly among 3 or more values
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What do you need to calculate in ANOVA?
The grand mean and group mean
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What is the F statistic ratio?
The between group variance / The within group variance
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What effect does a higher between group variance have on F?
Increases it
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What effect does a large amount of variance, due to chance, have on F?
Decreases it
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What does a high F value suggest?
Moe significance and difference is not due to chance
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How do you calculate sum of squares?
Data point - mean ^2
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When do you use a Mann whitney U test?
When you're data is not normal and you are testing one variable
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What parametric test is the Mann whitney U test compared to?
T-test
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How do you display data for a Mann whitney U test?
Rank it in ascending order
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Whats the next step?
You compare the two data points in each rank and find the difference. Ua and Ub
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Are the differences separated between when the X column is bigger and then when the Y column is bigger?
Yes
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What is the U value?
The ratio of Ua to Ub
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when do you use a Kruskall wallis test?
When the assumption of ANOVA are not met. Data is not normally distributed and there are not equal variances
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What is the null hypothesis?
The sample are from the same population
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Does Kruskall wallis involve ranking data too?
Yes
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What is the test statistic?
H
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What is the kruskall wallis test used for?
When two factors are being tested
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What is the limitation of the kruskall wallis test?
It only tells you that there is a difference or not between two groups. It doesn't tell you which groups
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What does two-way ANOVA test?
It compares the mean difference between groups that have multiple factors
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What types of variables must two-way ANOVA include?
Two nominal variables and one measurement variable
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Wat do you test?
What effect each nominal variable has on the measurement variable
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When would you transform data?
When it is not normally distributed
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How do you transform data?
Apply a log transformation. Base 10 (log10)
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Are you able to do statistical tests on transformed data?
Yes
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What does a correlation (r) measure?
The extent to which two variables change together
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What is the measurement of a correlation?
The correlation coefficient r
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What does r range from?
-1 to +1
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What would -1 and +1 tell you about the line?
Its a completely straight line where all points are on the line
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What doe a correlation look for?
A linear effect
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What unit does r have?
No units
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How do you calculate a correlation?
Calculate the sum of squares for the Y and X values then calculate the sum of cross products by adding them together
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What is r^2?
A coefficient of determination
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How many samples does a test need t give a significant r?
3
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What is regression?
It calculates the 'best' straight line through the plots in order to express the relationship of the correlation coefficient
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What is the regression line?
A line that minimises the sum of squared deviation of the plots from the line
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What is the intercept symbolised as?
c
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What is the intercept?
The point at which the line crosses the x-axis
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What is the slope symbolised as?
B
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What is the slope?
The gradient of the line
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How do you calculate the intercept?
The mean of Y - (mean X x b)
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What statistical test do you use to test the significance of regression?
Pearsons correlation
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Can you use not normally distributed data in regression?
No you must first transform the data
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What do yu do if the data does show relationship but its not linear?
Use a spearmint rank correlation test
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What type of data is linear models used on?
Normal data
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What type of data is generalised linear models used on?
Non-normal data/ count data
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Other cards in this set
Card 2
Front
How is probability expressed?
Back
A number (P-value)
Card 3
Front
What is the maximum and minimum value probability can be?
Back
Card 4
Front
What does the sum of probabilities for all outcomes equal to?
Back
Card 5
Front
How can you calculate probability?
Back
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