Statistics Flashcards
Normal distribution
Describes the probability of getting a certain value in a population.
Symmetric around the mean and the median.
Doesn’t change with a change in sample size
What portion of the population is represented by mean +/- 1 standard deviations
Mean +/- 1 standard deviation = 68% of population
What portion of the population is represented by mean +/- 2 standard deviations
Mean +/- 2 std deviation= 95% of the population
What portion of the population is represented by mean +/- 3 standard deviations
99.7%
What can you construct using normal distribution
Reference interval
Standard error
Not a measure of variability in population.
It is the standard deviation of the sampling distribution. Measures the precision of the estimate (i.e how reliable is our mean)
Inversely proportional with sample sizes.
Standard error - quantifies the variation in means from multiple sets of measurements
Standard deviations - quantifies the variation within a set of measurements.
Define variance and standard deviation
Range, standard deviation and variance both measure the spread or variability of a data set i.e dispersion.
Range = biggest - smallest number
Variance and standard deviation have a close relation. Variance is the SD squared.
- average of the squared differences from the mean. Variance gives you a sense of outliers.
Standard deviation = square root of variance.
A measure of how spread out numbers are. Standard deviation is more proportionate to average distance from the mean.
Define sensitivity
Proportion of people with the disease who test positive
Probability of a positive test given you have the disease
Define specificity
Proportion of people without the disease who test negative
Probability of a negative test given you don’t have the disease
Define PPV
Of those with a positive test how many have the disease.
Not an intrinsic property of the test itself, influenced by prevalence of disease.
Define NPV
Of those with a negative test, how many don’t have the disease
Define accuracy
How close a given set of measurements (observations or readings) are to their true value.
What is a continuous diagnostic test
A test which gives a continuous measure.
We determine where to put cut off (somewhat arbitrary).
If we alter the cut off we change sensitivity or specificity.
Increase the value - increases specificity and decreases sensitivity
What is a sampling distribution
The distribution of that statistic, considered as a random variable, when derived from a random sample of size.
How do you calculate the SE
SE = Standard deviation / square root of sample size
If there is no true difference between populations then what is the mean
0
What is the P value
The probability we would observe a difference in the sample means this large or larger, just by chance/ if there were no true difference
Define the null hypothessis
No true difference.
Define the alternative hypothesis
True difference