Use this as an introduction to logistic regression using the transition from the chi-square test. Note that the chi-square test performs an overall hypothesis test of whether physician proportion of VTE prophylaxis, but it does not tell you which physicians are necessarily significantly different.Ask the question about whether physician 8 is significantly different than physician 3In this example, physician 3 was chosen as the reference physician since they have the highest rate. You can choose any physician you like, but I suggest choosing the highest for comparison.Logistic regression not only performs an overall test of hypothesis about whether all the proportions are equal, but it conducts a hypothesis test of each variable (physician in this case) and produces a p-value. Therefore, you are able to ascertain which physicians are significantly different from the reference physician. Used the Surgical_VTE.MPJ dataset for this example.
Note that chi-square response variable is not necessarily limited to a binary variable.Minimum sample size rule of thumb is 10 observations per explanatory variable in the model.
The null hypothesis states that none of the explanatory variables have a statistically significant effect on the response variable
The mean response of the binary logistic regression model is the probability that Y=1 (Neter page 568). Or put another way it represents the proportion of observations (patients) that have the outcome of interest (Hosmer/Lemeshow page 20)Odds Ratio = eB1. e = exponentiate (2.718) the coefficient
It is a measure of association. It quantifies how much more likely (>1) or unlikely (<1) it is for the outcome to be present among those with x=1 than among those with x=0.The odds ratio = eCoef and ln (odds ratio) = Coef: when the independent variable is coded as 0 and 1.The odds ratio confidence intervals are derived as Coef ± 1.96(SE Coef) then exponentiate (eCoef) the end points.e = 2.71828183
The reference category has an odds ratio of 1 (HosmerLemeshow page 57).
The class should perform the data screening prior to conducting the analysis.Note that in the previous analysis we discovered that receiving and interpreting an initial ecg with 10 minutes significantly improved the chance of receiving a thrombolytic within 30 minutes. So the question becomes: what factors effect time to initial ECG?The QI team now can begin to analyze factors that effect whether an initial ecg is conducted and evaluated within 10 minutes.
Potentially hook up to one person’s computer and review results. Shift 3 actually has a univariate higher proportion, but the odds ratio is <1 in a multivariate environment. If you conduct the analysis with shift as one variable, the shift 1 and 2 have odds ratios <1, but when add other variables it changes – especially with the addition of the dedicated ecg tech variable. Implying that use of the ecg tech may be inconsistent across shifts.Ask attendees to explain parts of the output. The overall null hypothesis is rejected. Overall the model fits the data.QI Implications: Investigate the following: Why is there a difference in sex and race? Why doesn’t the ecg protocol have a significant impact on ecg within 10 minutes? Why don’t patients with chest pain have a significant impact on getting an ecg within 10 minutes?Looks like need to use dedicated ecg tech more consistently and review and or revise the protocol and train clinicians on the protocol. Understand either if the protocol is not being used or if it being used incorrectly.
Potentially hook up to one person’s computer and review results. Shift 3 actually has a univariate higher proportion, but the odds ratio is <1 in a multivariate environment. If you conduct the analysis with shift as one variable, the shift 1 and 2 have odds ratios <1, but when add other variables it changes – especially with the addition of the dedicated ecg tech variable. Implying that use of the ecg tech may be inconsistent across shifts.Ask attendees to explain parts of the output. The overall null hypothesis is rejected.QI Implications: Investigate the following: Why is there a difference in sex and race? Why doesn’t the ecg protocol have a significant impact on ecg within 10 minutes? Why don’t patients with chest pain have a significant impact on getting an ecg within 10 minutes?Looks like need to use dedicated ecg tech more consistently and review and or revise the protocol and train clinicians on the protocol. Understand either if the protocol is not being used or if it being used incorrectly.
