Interpretation of results depends on
The study design
The study design should be tailored to..
..the research question
Methods of statically analysis and information produced will depend on
The study design
Two main types of study design
- descriptive studies
- analytic studies
Descriptive studies
- studies which describe things
E.g surveys - descriptive studies are often simply referred to as surveys
- generally use a sample from the population of interest
Analytic studies
- studies which test hypotheses
Experimental studies e.g. randomised controlled trials
Observational studied e.g. cohort studies; case-control studies
What is a population
- a complete set of entities or elements or units people that we wish to describe or make inference about
Well-defined population:
- the collection of poems by W.B Yeats
Not well-defined population:
- the population of NZ
What is a sample
- a subset of a population
How to provide a ‘ representative’ sample of the population
- random sampling (random selection) of study units os important
What is random sampling is impossible ?
- need to consider carefully how likely it is that our sampling procedure will produce a repressentiative sample
- sometimes we can compare the characteristics of the sample to known facts about the population
Sampling frame
- list of items in a population from which a sample is drawen
- rearely coincides with the entire population of interest
- often doesn’t exist
Even without a list we can ensure an ‘unbiased’ sample if every individual has the known chance of being drawn
Random sampling to obtain a subgroup
- choose the sample in such a way that every individual in the population has a known change of being selected
- in a simple random sample, everyone has an equal chance of being chosen
- this method is the best way of obtaining a sample which is represtiticve of the population
Sample mean equation
Sample mean = population (true) mean + error
Error could be systematic or random
Random error occurs due to…
… natural variability
In regards to random error… increasing the sample size will….
Reduce the random fluctuations in the sample mean
Statistical methods allow us to quantify the influence of random error on our estimate
Two types of error
- Random
- systematic
Systematic error in a destructive study (bias) occurs due to..
Aspects of the design or conduct of the study which systematically distort the results
Systematic error in a descriptive study can/cant be reduced by increasing sample size
Can’t be reduced by increasing the sample size
Two types of bias that cause systematic error in a descriptive study
Selection bias - if a sample is not representative of the population
Information bias - if the information collected from the sample members is incorrect
We want out sampling frame to..
Match the population of interest
Probability sampling is important because..
It helps justify the statistical models which roll be introduced in this course
The key characteristic of probability sampling is..
That we know the probability of being selected for everyone in the sample frame
Simple random sample
- the simplest form of probability sampling
- for a finite population of size N draw a sample of size n such that each possible sample has the same probability of being selected
Simple random sample
