90288 – Select a Sample and Make Inferences from Data The Mayor’s Claim

Choosing a Sampling Method I am going to use a Stratified Sampling Method 1. Firstly I will stratify the population into districts and number them accordingly. Central 1-100, Darby 1-20, Appleton 1-20 and Beachhead To calculate the right proportion of districts for my sample I will use: Number in district x 30 i.e. 100 x 30 = 15 Total in population 200 I will need 15 Central, 3 Darby, 3 Appleton and 9 Beachhead houses 3. To pick my sample I will use the random number on my calculator. I will use the formula: 100Ran#+1 to pick the 15 from Central, 20Ran#+1 to pick 3 from Darby etc 4. I will ignore any repeats (important to include this) 5. I will relate the random numbers back to the population to pick the 30 houses for my sample. 6. I will then calculate the appropriate statistics to reject/accept the mayor’s claim.

Listing the data gathered Include all information given for the population members (incl. list numbers) District IncreaseDistrict Increase Appleton Central Appleton Central Appleton Central Beachhead Central Beachhead Central Beachhead Central Beachhead Central Beachhead Central Beachhead Central Beachhead Central Beachhead Central Beachhead Central Central Darby Central Darby Central Darby

Calculating Statistics An average: e.g. Mean = $3267 It would also be wise to calculate the Median as well. A measure of spread e.g. Standard Deviation = $10780 Other statistics can be calculated but you should at least list the 3 above.

Making an Inference Remember the key words to use in your inference: e.g. I predict the population mean house price increase to be approximately $3300 Then make sure if you have answered the actual problem e.g. From my prediction, I believe that the Mayor’s claim was too high.

Justifying Choice of Sampling Method It is best to justify the use of the Stratified Sampling Method e.g. I have chosen this method as it will enable me to obtain the correct proportion of all four districts into my sample. I will also be selecting the houses by random sampling so every house still has an equal chance of being selected. This should give me a sample that is representative of the population and not bias in any way.

Is the Sample Representative? Remember it is OK if you don’t think your sample is representative! e.g. I think that the sample I have chosen is representative of the population. Each of the four districts are fairly represented in the correct proportions and within each district I have a good range of house prices. Because of this reason I believe that my sample averages would make accurate predictions for the population and therefore reject the mayor’s claim. My sample did contain a couple of high price increases from Appleton and if I was to ignore these values my sample mean would most likely be even lower. To be totally sure of my prediction however, it may have been an idea to use the median for my prediction as this would not have been influenced by extreme values.

Evaluating Your Processes Improving your Sampling Method Limitations of your sampling and statistical processes Limitations of your conclusion Accuracy or appropriateness of your estimate Distribution of the data It is always worthwhile to look to discuss way to improve things as it is unlikely your methods used were perfect. It is ok to say you could’ve done things better. Remember to always write more statements than that required.