Subtitles section Play video Print subtitles Choosing which statistical test to use. There are many different tests you can use in statistics. Sometimes it can be quite difficult to know which is the correct test to use. This video will talk about seven tests you are likely to use. involving means proportions and relationships. When you are trying to work out which is the most appropriate test there are three questions you should ask One. What level of measurement was used for the data we are analyzing. 2. How many samples do we have? 3. What is the purpose of our analysis? I will now explain each of these questions 1. Data or level of measurement Is our data nominal or interval/ratio? Nominal data is also called categorical, qualitative or nonparametric Examples of nominal data are color whether parts are defective or not, or preferred type of chocolate. Nominal summary values are usually stated as frequencies, proportions or percentages. The tests that involve nominal data are: Test for a proportion Difference of two proportions and chi-squared test for independence The other type of data is interval/ratio also called quantitative Examples of interval/ratio data are daily sales figures for choconutties weight of peanuts or temperature the most common summary value for interval/ratio data is a mean. Tests that involve interval/ratio data are: Test for a mean difference of two means - independent samples difference of two means - paired and regression analysis. For more help on levels of measurement see our video: "Types of data nominal, ordinal, interval/ratio" Ordinal data can be classified with nominal or interval/ratio depending on the circumstances. 2. Samples Next we ask how many samples are involved Is there one sample for which we are testing the relevant statistic against a hypothesized value or are there two samples which are being compared with each other or is the one sample but each observation has a measure or score for more than one variable? The same sample is measured twice. If we wish to compare a proportion or a mean against a given value, this will involve one sample. If we're comparing two different lots of people or things such as men and women or people from two different departments then we would have two samples. If we have two sets of information on the same people of things we would say we have one sample with two variables. An example is one set of days and information on how many choconutties are sold and what the temperature was. Or - one set of people and information on their gender and preferred type of chocolate. Finally we ask What is the purpose of the analysis? We can be testing against the hypothesized value comparing two statistics or looking for a relationship. Chi-squared test for independence and regression are similar in that they are looking at the relationship between two variables The difference between them is in the kind of data. If you would summarize the data in s table, we would use a chi-squared test fo independence whereas if you would put it on a scatter plot you would use regression analysis. Here iss an example for each of these tests. They relate back or out other videos teaching about hypothesis testing. After each description of the scenario pause the video and see if you can identify the correct test before we tell you the answer. Helen is still selling choconutties. Example one: sufficient nuts. Helen was concerned whether the quantity of nuts was sufficient in her choconutties. She took a sample of twenty packets and found the weight of nuts in each packet Pause the video 1. Data The weight was interval/ratio data. 2. Samples There was just one sample of twenty packets of choconutties. 3. Purpose. Helen was comparing against given value Thus, the test she needs to use is Test for a mean. Example Two Prize tickets In a promotional campaign twenty percent of all packs of choconutties should include tickets for free prizes. Helen takes a sample of fifty packets and finds that seven of them have winning tickets Pause the video 1. Data: For each bar we are saying yes or no, only to be lumped whether or not there is a ticket. This is nominal data from which we get a sample proportion of seven out of fifty Or 0.14 Samples There is one sample of fifty packets Purpose. Helen is comparing the sample value against a given value: twenty percent We conclude that the test she needs to use is test for a proportion. Example three Bar longevity compared with nuttabars. Helen thinks her choconutties last longer than the competition, nuttabars. She gets 36 people to eat one of each, and records their eating times. Pause now 1. Data. Helen collects times taken in seconds so this is interval/ratio data. 2. Samples There is one sample of thirty-six people but with two scores for each person the time for the choconuttie and the time for the nuttabar. 3. Purpose She is looking at whether there iss a difference in the amount of time taken for each of the bars. Thus the test is difference of two means, paired sample. Example four Defective wrapping from two wrapping machines Helen thinks there is a difference in performance between the two wrapping machines in her factory. She checks 200 bars from one machine and 150 bars from the other. For each bar she is seeing if the wrapping is satisfactory or not She finds that ten out of two hundred bars from the first machine and nine out of 150 bars from the second machine are badly wrapped. Pause the video Data. The information for each bar is OK or not ok This is nominal data. It has been summarized as frequencies. 2. Samples there are two independent samples one sample from each of the two machines 3. Purpose Helen is comparing the proportions from the two samples We can see that the test is difference of two proportions. Example five Do stickers help sales? Helen is exploring whether having free stickers makes a difference to sales. She has the sales figures for thirteen days when she did offer free stickers and ten days when she did not. Pause and decide on the test Data. For each day Helen has a number or value corresponding to the sales for that day This is interval/ratio data It is summarized as a mean member of sales. 2. Samples There are two samples one sample for days with stickers and one sample for days without. 3. Purpose Helen is comparing the average sales figures for the two treatments we conclude that the test to use is... Difference of two means independent samples Example six Are sales affected by temperature? Helen wants to see if there is a relationship between the daily temperature and sales of choconutties. She has data on sales and temperature for thirty weekdays of sales Pause! Data. Sales and temperature at both interval variables Samples There is one sample of thirty days with two measures or scores for each day. Purpose. Helen is interested in the relationship between sales and temperature This leads us to decide that the test is regression. Example seven Men and women and chocolate preference Helen is thinking of selling dark chocolate, milk chocolate and white chocolate choconutties. She thinks that men and women might have different preferences with regard to type. She collects data from fifty customers, noting down if they are men or women and asking them which variety they prefer. Pause the video and decide. Data. Helen