## Degrees of freedom and t tables

Table 2-1 is an abbreviated t score table that shows the values of t corresponding to different areas under the normal distribution for various sample sizes. Tables of t values do not show sample size (n) directly instead, they express sample size in terms of degrees of freedom (df). For the purposes of USMLE, degrees of freedom (df) can be defined as simply equal to n 1. Therefore, ro determine the value of t (such that 95 of the population of t-statistics based on a sample size of 15 lies...

## Negative predictive value

The negative predictive value (NPV) of a test is the proportion of negative results that wfi* are true negatives, i.e., the likelihood that a person with a negative result truly does not - have the disease py _ number who test negative and do not have the disease (TN) total number who test negative (FN + TN) Knowing a test's NPV allows one to answer the question, Given that the test result is negative, how likely is it that the disease really is absent Once again, this is the kind of...

## Statistics in Epidemiology

Epidemiology is the study of the distribution, determinants, and dynamics of health and disease in groups of people in relation to their environment and ways of living. The basic statistical measures in epidemiology are rates and measures of risk. All rates consist of a numerator (usually the number of people with a particular condition) and a denominator (usually the number of people at risk), and they usually specify a unit of time. The most important rates are incidence and prevalence (which...

## Casecontrol studies

Whereas cohort studies examine people who are initially free of the disease of interest, case-control studies compare people who do have the disease (the cases) with otherwise similar people who do not have the disease (the controls). Case-control studies start with the outcome, or dependent variable (the presence or absence of the disease). They then look back into the past for possible independent variables rhat may have caused the disease, to see if a possible risk factor was present more...

## Exercises

Select the single best answer to the questions referring to the following scenario. A family physician is interested in the cigarette use of patients in her practice. She asks all patients who come into her office if they use cigarettes and determines that 20 of her patients smoke. She then asks every third smoker who comes to the office how many cigarettes they smoke each day she finds that the mean number of cigarettes smoked is 16. She plots the number of cigarettes smoked by each patient on...

## Correlational Techniques

Biomedical research often seeks to establish if there is a relationship between two variables for example, is there a relationship between salt intake and blood pressure, or between cigarette smoking and life expectancy The methods used to do this are correlational techniques, which focus on the co-relatedness of the two variables. There arc two basic kinds of correlational techniques Correlation, which is used to establish and quantify the strength and direction of the relationship between two...