When we test our SuperPaste (for preventing dental caries), we need special counting tools to measure the dental problem. These tools tell us the Disease Frequency (how often the disease appears).
1. The Population at Risk
Before counting, we define who can actually get the disease.
- Population at Risk: This is the portion of the general population that is susceptible to a disease.
- SuperPaste Example: If we are testing for dental caries in kids, the population at risk would be children aged 6–12 with existing teeth. If we were studying carcinoma of the cervix, the population at risk would only be females, usually in a specific age range.
2. Prevalence (Existing Cases)
Prevalence is like taking a snapshot right now. It is the measure of existing cases (both old and new) at a specified point of time in our population.
| Terminology | What It Means in the Story | The Formula |
| Prevalence | The total number of kids who currently have dental caries (cavities) right now (using the DMFT index). | (Number of people with disease at specific time) / (Population at risk at specific time) |
| Point Prevalence | A measure of disease frequency at one specific moment. “Like a snapshot.” | Example: On Monday morning, we check 100 kids. 10 of them already have caries. The Point Prevalence is 10/100, or 10%. |
| Period Prevalence | A measure of disease frequency over a period of time (e.g., over a whole year). | Example: At the start of the year (C), 10 kids had caries. Over the year (I), 5 new cases developed. Period Prevalence is (10 + 5) / 100 = 15%. |
- Interpretation: Prevalence measures the burden of disease—it helps dentists know how many chairs and how much time they need to treat all the existing problems (planning health services).
- A Warning: Changes in Prevalence are difficult to interpret. If the prevalence of caries suddenly drops, it could mean the new SuperPaste worked (good!), or it could mean all the kids with bad caries moved away (unrelated!).
3. Incidence (New Cases)
Incidence is like watching a movie of new problems appearing over time. It measures the rapidity or speed at which new cases develop in a population that was healthy at the start. Time must be specified (e.g., per month, per year).
| Terminology | What It Means in the Story | The Formula |
| Cumulative Incidence (Risk) | The risk (probability) that a healthy kid will develop a new cavity over our follow-up period (e.g., 1 year). | (Number of new cases in a time period) / (Population at risk at the beginning) |
| Incidence Density (Incidence Rate) | A more exact measure. It accounts for the person-time of observation. | (Number of new cases) / (Total person-time of observation) |
- Person-Time: If we follow 100 kids for one year, that is 100 person-years. But if 10 kids leave after 6 months, we only observed them for half a year (5 person-years). Incidence Density is better because it accounts for people entering or leaving the study.
- Interpretation: Incidence is used to see if our primary prevention programs (like giving them SuperPaste) are actually preventing new problems. If the incidence is low, SuperPaste is working!
4. The Relationship (P = I x D)
Prevalence and Incidence are related, especially for stable, chronic conditions like dental caries:
$$\text{Prevalence} = \text{Incidence} \times \text{Duration}$$
- If the Incidence (new cases) is high, the Prevalence (existing cases) will also be high.
- If the Duration of the disease is very long (like a cavity that lasts a lifetime), the Prevalence will build up over time.
- Example: Common cold has Low Prevalence and High Incidence. It happens fast, but you cure fast.
- Example: Dental Caries can have High Prevalence and Low Incidence. The rate of new cavities might be low, but the total number of existing, old cavities in the population (the prevalence) is always high because they last a long time if untreated.
5. Case Fatality
This measure is used for serious diseases where death is an outcome, though less common in dentistry unless studying certain oral cancers or infections.
- Case Fatality: The proportion of deaths from a disease compared to the number of people who had the disease.
- Interpretation: It reflects the severity of the disease. A disease with high case fatality is more deadly.
Leave a Reply