2. Statistical inference

Question 1

What is a sample?

Answer 1

SAMPLE
- A subset on units
- From a population of interest
- Used to estimate a population parameter

Question 2

What is a random sample?

Answer 2

RANDOM
- Every unit in the population must have an equal chance of being sampled
- Selection of units must be independent

Question 3

What is a sample of convenience?

Answer 3

A collection of units that are easily available to research
- E.g. volunteers with a predefined set of traits.

Limits how much the sample reflects the population

Question 4

What is sampling error?

Answer 4

The deviation of an estimate from its population parameter

Question 5

What is bias?

Answer 5

A systematic departure of the estimate obtained by sampling and the true population parameter

Question 6

What is precision?

Answer 6

The spread of estimate resulting from sampling error

Question 7

What is accuracy?

Answer 7

The extent to which an estimate reflect a sampling bias

Question 8

What is a frequency distribution?

Answer 8

Histograms
- that can be used to display discrete frequency distribution data
- using actual data from a sample

Question 9

What is a probability density function?

Answer 9

It estimates data from the whole population

Question 10

What are the main measures of Central Tendency on a normal/gaussian distribution?

Answer 10

Mean
- Sample mean notation: xbar
- Population mean notation: μ

Median

Mode

Question 11

What are measures of distribution spread?

Answer 11

Range
Variance
Standard deviation

Question 12

What is range?

Answer 12

The difference between the maximum value (Xmax) and the minimum value (Xmin)

Question 13

What is variance?

Answer 13

The sum of squares over the sample size/population

Measures distance of the deviation from the mean

Question 14

What is standard deviation

Answer 14

This puts the distance of the deviation from the mean back into its original unit

root of variance

Question 15

How do you calculate the sample variance and standard deviation?

Answer 15

Sample variance
S^2 = sum of ((difference of value from mean)sqrd) over (sample-1)

Sample deviation
S = √(S^2)

Question 16

How do you calculate the population variance?

Answer 16

same as sample variance but divide by population size rather than (sample-1)

N instead of n-1

Question 17

Why is n-1 used?

Answer 17

reflects the variance of the sample better by reducing bias in the estimate

Question 18

How do you calculate the standard error?

Answer 18

sample deviation (s) over root sample size (√n)

Question 19

How do you calculate the CV?

Answer 19

Sample deviation (S) over mean value (Y-bar)

Question 20

How do you calculate the Interquartile range?

Answer 20

The difference between the third quartile and the first quartile

• if 0.25n or 0.75n is an integer, you are able to take the average of the value and the one ahead of it

Question 21

What is a sampling distribution?

Answer 21

the skew (or lack of ) of data

skewed distributions show asymmetry towards one direction
(Median tends to be better for skewed frequency distributions)

data may be skewed due to bias/error/ or a small sample size

Question 22

How do you calculate a 95% confidence interval?

Answer 22

mean ± 2(SE)