In my ongoing series that reviews epidemiological concepts for the comps, I continue to blog about **Introductory Epidemiology** (EPI 810) concepts. Today, I am going to use Gordis text *Epidemiology* and review important concepts chapter-by-chapter. I’ll start with Ch 2 and only try to cover those concepts that either I don’t remember or bear repeating.

Chapter 2 – The Dynamics of Disease Transmission

- Disease severity represents a broad spectrum.
**Clinical disease**is the common stage of disease that shows signs and symptoms. With a**nonclinical (inapparent) disease**state, the disease is not clinically apparent (i.e., no signs or symptoms) and may never arise to become clinically significant, but it is often diagnosed by a biologic test.**Persistent (chronic) disease**is one in which the symptoms continue over a long period of time.**Latent disease**is usually characterized by a viral infection that lies dormant (doesn’t replicate) and does not produce disease. **Herd immunity**is a concept that essentially means immunity conferred to a group of people once a certain proportion of the population is immune (through natural immunity, vaccination, cured). A susceptible person is unlikely to contract the disease if he/she is only coming into contact (or is more likely to come into contact) with immune individuals.- The
**incubation period**is defined as the time between the contraction of a disease and the onset of clinical illness (i.e., signs and symptoms). - An
**attack rate**is the number at risk in whom a certain illness develops divided by the total number of people at risk. Time is not explicit, but implicit. - The
**secondary attack rate**is the attack rate in susceptible people who have been exposed to the primary case.

Chapter 3 – Measuring the Occurrence of Disease: I. Morbidity

- Remember that an
**incidence rate**is the number of new cases of a disease occurring during a specific time period divided by the number of persons at risk of developing the disease during the same time period. Understand the concept of person-time (the sum of the time periods of observation for each person). - The difference between
**active**and**passive surveillance**. Active surveillance is where study staff go out to sites to identify new cases. Passive surveillance is where new cases are found through either available data or reported by health care providers.

Chapter 4 – Measuring the Occurrence of Disease: II. Mortality

- The
**annual mortality rate**is the total no. of deaths divided by the total population at midyear. - The
**case fatality rate**is the no. of people dying of a disease during a specific period of time divided by the total no. of individuals with that disease. - Know how to do
**direct- and indirect- age adjustment**. With the direct-adjustment you are basically applying the population-specific mortality rate(s) to determine the expected number of deaths, and then dividing the total expected deaths by a ‘standard’ population. See Table 4-11 (pp.76) for a good illustration. - For indirect-age adjustment one can calculate the
**standardized mortality ratio (SMR)**, which is the ratio of the observed no. of deaths per year / expected no. of deaths per year. See Table 4-13 (pp. 78) for a good illustration.

Chapter 5 – Assessing the Validity and Reliability of Diagnostic and Screening Tests

**Validity**of a test is the ability of the test to distinguish between who has the disease or not. It has two components: sensitivity and specificity.**Sensitivity**is the ability of the test to correctly identify true cases of the disease. It is the ratio of the true positives (people with the disease that tested positive) divided by the total number of people with the disease.**Specificity**is the ability of the test to correctly identify true non-cases of the disease. It is the ratio of the true negatives divided by the total number of people without the disease.- When tests are used sequentially, net sensitivity
**decreases**, while net specificity**increases**. - When tests are used simultaneously, net sensitivity
**increases**, while net specificity**decreases**. - Remember that the
**positive predictive value**of a test asks, “If we screened the population, what proportion of people who have the disease will be correctly identified?”. It is the number of true positives divided by the total number of positives. - Conversely, the
**negative predictive value**of a test is the number of true negatives divided by the total number of negatives. - What is the relationship between prevalence and the predictive value of a test? The higher the prevalence, the higher the predictive value.
- Specificity has a bigger impact on the positive predictive value than sensitivity. As specificity increases, the PPV increases.
**Reliability**of a test is whether it is repeatable. There are different types of variation in reliability for tests.**Intrasubject**variation within the individual (e.g., blood pressure).**Intraobserver**variation is differences in observation by the same observer.**Interobserver**variation is differences between two observers.**Kappa**is the (% agreement observed – % agreement by chance) / (100% – % agreement by chance). See Figure 5-17 (pp. 105) for an illustration.

Chapter 6 – The Natural History of Disease: Ways of Expression Prognosis

- The two main points to understand about this chapter is calculating a life table using the standard and the Kaplan-Meier method, and then understanding a few assumptions of the life table.
- I will return to this post and create an example for each at a later date.

I am going to stop here for today.