# Ordinary Regression - Assumptions

## Learning Objectives

After completing the activities this module you should be able to:

• Analyze data using linear regression
• Transform data to have residuals that have a Normal distribution
• Interpret output of ordinary regression
• Test null hypothesis that a regression model has no fit with the data
• Test null hypothesis that an independent variable is related to the dependent variable

## Assignments

Assignments should be submitted in Blackboard. The submission must have a summary statement, with one statement per question. All assignments should be done in R if possible.

Question 1: Clean the Medical Foster Home data. Limit the data to cost per day, patient disabilities in 365 days, survival, age of patients, gender of patients and whether they participated in the medical foster home (MFH) program. Clean the data using the following:

1. Remove all cases in which all values for disabilities in 365 days, age and gender, are missing.  These are meaningless data and should be dropped from analysis.
2. Remove any row in which the treatment variable (MFH) is missing.  MFH is an intervention for nursing home patients.  In this program, nursing home patients are diverted to a community home and health care services are delivered within the community home.  The resident eats with the family and relies on the family members for socialization, food and comfort.  It is called "foster" home because the family previously living in the community home is supposed to act like the resident's family. Enrollment in MFH is indicated by a variable MFH=1.  A value of NaN or null is missing value.
3. Various costs are reported in the file, including cost inside and outside the organization.  Rely on cost per day. Exclude patients who have 0 cost per day within the organization. These do not make sense. The cost is reported for specific time period after admission, some stay a short time, and others some longer.  Use daily cost so you do not get caught on the issues related to lack of follow-up.
4. Select for your independent variables the probability of disability in 365 days.  These probabilities are predicted from CCS variables.  CCS in these data refer to Clinical Classification System of Agency for Health Care Research and Quality.  CCS data indicate the comorbidities of the patient.  When null, it is assumed the patient did not have the comorbidity.  When data are entered it is assumed that the patient had the comorbidity and the reported value is the first (maximum) or last (minimum) number of days till admission to either the nursing home or the MFH. Thus an entry of 20 under the minimum CCS indicates that from the most recent occurrence of the comorbidity till admission was 20 days.  An entry of 400 under the Maximum CCS indicates that from the first time the comorbidity occurred till admission was 400 days. Because of the relationship between disabilities and comorbidities, you can rely exclusively on disabilities and ignore comorbidities.
5. Check if cases repeat and should be deleted from the analysis.
6. Convert all categorical variables to binary dummy variables. For example, race has four values W, B, A, Other, and null value.  Create 5 binary dummy variables for these categories and use 4 of them in the regression.  For example, the binary variable called Black is 1 when race is B, and 0 otherwise.  In this binary variable we are comparing all Black residents to non-Black residents that include W, A, null, and other races.
7. In all variables where null value was not deleted row wise, e.g. race being null, the null value should be made into a dummy variable, zero when not null and 1 when null.  Treat these null variables as you would any other independent variable.
8. Gender is indicated as "M" and "F"; revise by replacing M with 1 and F with 0.
9. Make sure that no numbers are entered as text
10. Visually check that cost is normally distributed and see if log of cost is more normal than cost itself. If a variable is not normally distributed, is the average of the variable normal (see page 261 in required textbook)?  Visually check that age and cost have a linear relationship.
11. Regress cost per day on age (continuous variable), gender (male=1, Female=0), survival, binary dummy variables for race, probabilities of functional disabilities, and any null dummy variable you have created.
12. Show which variables have a statistically significant effect on cost.  Does age affect cost? Does MFH reduce cost of care?

Question 2: Regress Circulatory Body System factor on all variables that precede it (defined as variables that occur more before than after circulatory events).  In the attached data, the variables indicate incidence of diabetes (a binary variable) and progression of diseases in body systems. You can do the analysis first on 10% sample before you do it on the entire data that may take several hours.

1. Check the normal distribution assumption of the response variable. Drop from analysis any place where the dependent variable is missing.  If the data is not normal; transform the data to meet normal assumptions. For each transformation show the test of Normal distribution. You should at a minimum consider the following transformations of the data:
• Odds to probability transformation
• Log of odds to probability transformation
• Logarithm transformation:
• Third root of odds
2. Check the assumption of linearity.  If the data have non-linear elements, transform the data to remove non-linearity.

Question 3:  If you see the following residual versus fitted, distribution, and QQ plot after fitting a linear regression to the data, which of the following statements are true: (a) there is a linear upward trend, (b) there is a linear downward trend, (c) there is curved upward trend, (d) there is a curved downward trend, (e) there is a fanned shaped trend:

Question 4: If you see the following residual versus fitted, distribution, and QQ plot after fitting a linear regression to the data, which of the following statements are true: (a) there is a linear upward trend, (b) there is a linear downward trend, (c) there is curved upward trend, (d) there is a curved downward trend, (e) there is a fanned shaped trend:

Question 5:  The attached data comes from Larry Hatcher's book titled "Advanced Statistics in Research"  The response variable is average of grades in graduate school and the independent variables are listed in the following table are admission information:

1. What does adjusted R2 measure and "according to the criteria recommended by Cohen (1988), does this value of R2 come closest to representing a zero effect, a small effect, a medium effect, or a large effect?"
2. "According to Table E9.3.2, the unstandardized multiple regression coefficient for undergraduate GPA overall is b = 0.313. What does this value mean? In other words, what is the correct interpretation of this coefficient (hint: your answer must incorporate the definition for an unstandardized multiple regression coefficient, and it must also incorporate the value “0.313”)."
4.  "What was the unstandardized multiple regression coefficient for undergraduate GPA overall?"
5. "What was the 95% confidence interval for this coefficient?"
6. "Assume that you had access to this 95% confidence interval but not the p value for the regression coefficient. Based only on this 95% confidence interval, was this regression coefficient statistically significant? How do you know?"
7. Is the statistical test of the coefficient for undergraduate GPA's affected by other variables in the model?  How is the test of coefficient in a regression model different from hypothesis testing of the same variable by itself and without other variables in the regression model.  Explain your answer.
8. "If an applicant increased his or her GRE quantitative test score by one standard deviation, that applicant’s score on graduate GPA overall would be expected to increase by how many standard deviations (while statistically controlling the remaining predictor variables)?"
9. "What percent of variance in graduate GPA overall is accounted for by the GRE verbal test, above and beyond the variance already accounted for by all of the other predictors?"
10. Is the GRE verbal test more useful in predicting graduate grades than the GRE quantitative test?
11. Does the model capture interaction among the variables?