This week, we are looking at the relationship between quantitative variables. We will concentrate on two variables for the purpose of this Discussion. We will be looking at the direction-positive or negative-and the strength-weak, moderate, or strong-of the correlation. There are no hard and fast rules on what numbers mean weak, moderate, or strong correlations; however, here is a rule of thumb:
0 < 0.4 weak 0.4 < 0.6 moderate 0.6 - 1 strong, where the numbers can be positive or negative. Remember that you can only "prove" that one variable "causes" another by using an experiment. Be skeptical of observational studies that claim causation. To prepare for this Discussion: Review the following resources: Discussion Resources (click to expand/reduce) Discussion Videos Finding Correlation Using Statdisk (4m) Statdisk to Find r (2m) StatCrunch: Creating Scatterplots (5m) Discussion Documents Scatterplots(PDF) Correlation(PDF) Look at the data set that you used in Discussion 1 and/or 2. In order to do this week's Discussion, you will need to have two quantitative variables that you think might be related. In other words, changes in one variable result in changes in the other variable. If you do not have two variables like this in your data set, add another variable. Example: (click to expand/reduce) I will go back to the original data set I used in the Week 1 Discussion. I think that a person's weight is related to his or her systolic blood pressure. Since the systolic pressure depends on the weight, it will be the dependent variable in Statdisk, and the weight will be the independent variable. My data set is below. GenderWeight (lb.)Systolic Pressure (mm Hg)M150144F110100M225140M250113M275128F12598F15094M235138F176128M267116M145132F13688M285126F178130F112100 Review the rubric that will be used for grading. With these thoughts in mind: By Day 1 By the end of Day 1, you can post your scenario and data for your Instructor to review. If you don't submit this by the end of Day 1, you will need to proceed to submit the entire Discussion on Day 3. By Day 3 Post a 1- to 2-paragraph write-up that includes the following: You will want to use Word for your Discussion post because the scatterplots will not post directly in the Discussion. Include your data set and describe the scenario you will be using for the correlation Discussion. Enter the data for your two quantitative variables in the Statdisk Sample Editor. Create a scatterplot for the data and paste it in your document. Discuss what the scatterplot tells you about the correlation (how strong is the correlation and what is the direction). Calculate the correlation coefficient using the Correlation and Regression option in Statdisk and paste your results into your document. Discuss the strength of the correlation (weak, moderate, or strong). Are there any other variables that might affect the relationship between the two quantitative variables? Describe a correlation study that you might conduct related to your field of study, aside from the one used in your Discussion.