stats

Question 1

define the median

Answer 1

the central value when data is placed in order (if even number it is halfway between the two middle values).

Question 2

when is it best to use the median

Answer 2

It is not affected by outliers, and reflects what most people experience so is useful when data isn’t skewed

Question 3

what is the mode

Answer 3

the most frequent value. It reflects what most people experience, is rarely used

Question 4

what is the range

Answer 4

the difference between the highest and lowest values

Question 5

what is the interquartile range

Answer 5

is where the data is divided into quarters (quartiles) and is the difference between the middle two quartiles. I.e. it is the middle 50% of values.

Question 6

what is standard deviation

Answer 6

the average distance of the observations from the mean value. It is used to find abnormal results or “outliers”

Question 7

describe a box and whisker diagram

Answer 7

The median is shown by a line across the main box.

The lines extend to the highest and lowest results excluding outliers.

The outliers are those values which are more than 1.5 x the interquartile ranges away from the upper or lower edges of the box. They are shown as dots.

Question 8

describe how data is distributed in a bell shaped curve

Answer 8

2/3rds of the data lies within 1 standard deviation of the mean

95% lies within 2 standard deviations of the mean.

The median and the mean will be the same in a normal distribution.

Question 9

if data is symmetrical on a bell shaped curve what values should be used to summarise the data

Answer 9

the mean and standard deviation should be used to summarise data

Question 10

If data is skewed what measurements should you use to summarise th findings

Answer 10

the median and interquartile ranges should be used.

Question 11

describe positive skew in terms of mode / median / mean

Answer 11

mode < median < mean

Question 12

describe negative skew in terms of mode / median / mean

Answer 12

mode > median > mean

Question 13

what is the purpose of a reference range

Answer 13

A reference range gives limits within which we would expect the majority of data to fall.

Question 14

what is the usual reference range for standard deviated data

Answer 14

2 standard deviations above or below the mean (95% of the data).

Question 15

what is the difference between a population and a sample?

Answer 15

population is all the individuals in which we are interested. The sample is a group within a population which we will study.

Question 16

why are larger sample sizes better

Answer 16

reduce the standard error of the mean

Question 17

are random samples always representative

Answer 17

no

Question 18

what is stratified random sampling

Answer 18

population divided into groups and then randomly sampled within those groups

Question 19

what is cluster sampling

Answer 19

rather than sample individuals, groups or clusters of individuals are samples.

Question 20

what is standard error

Answer 20

the standard deviation of all the sample means - Standard error is an estimate of precision. It provides a measure of how far from the true value the sample estimate is likely to be

Question 21

how do you calculate standard error

Answer 21

the standard deviation divided by the square root of the number in the sample

the larger the data the lower the error

Question 22

what is a confidence interval

Answer 22

a range of values so defined that there is a specified probability that the value of a parameter lies within it.

The 95% confidence interval is found between two standard errors above and below the mean