Descriptive Studies
ECOLOGIC STUDIES  
involves making comparisons between variables where the unit of analysis is aggregated data on the population level rather than on the individual level 
CROSSSECTION STUDIES 
all variables measured at a point in time 
CASE REPORT 
is a profile of a single individual; it includes qualitative descriptive research of the facts in chronological order 
CASE SERIES 
involves a small group of patients with a similar diagnosis 
RATIOS, PROPORTIONS, RATES

Ratios, proportions, and rates are commonly used measures for describing dichotomous data. The general formula for a ratio, proportion, or rate is: X/Y x 10z 
RATES 
a type of frequency measure where the numerator involves nominal data that represent the presence or absence of a healthrelated state or event 
RATIOS 
the values of x and y are distinct, such that the values of x are not contained in y. The rate base for a ratio is 100 = 1 
PROPORTIONS 
x is contained in y. A proportion is typically expressed as a percentage, such that the rate base is 102 = 100. 
Dichotomous data Divided or dividing into two parts or classifications.
CALCULATION RATES
DEFINITIONS 
CALCULATIONS 
Incidence rate is the number of new cases of a specified healthrelated state or event reported during a given time interval 
Incidence Rate= New cases occurring during a given time period/population at risk during the same time period multiplied by 10z 
Mortalilty Rate is the total number of deaths reported during a given time 
Mortality Rate = Deaths occurring during a given time period/ Population from which deaths occurred Multiplied by 10z 
PersonTime Rate When the denominator of the incidence rate is the sum of the time each person was observed 
Person Time rate= New cases occurring during an observationperiod/Time each person observed, totaled for all persons multiply by 10z 
Attack Rate It involves a specific population during a limited time period, such as during a disease outbreak. It is also referred to as a cumulative incidence rate or risk 
Attack Rate=New cases occurring during a shirt time period/Population at risk at the beginning of the time period multiplied by 100 
Secondary Attack Rate the rate of new cases occurring among contacts of known cases. 
SAR= New cases among contacts of primary cases during a short time period/(Populations at beginning of time period) (primary cases) multiplied by 100 
Point Prevalence he frequency of an existing healthrelated state or event during a time period. 
Point Prevalence= Existing cases of a disease or event at a point in time/total study population at a point in time multiplied by 100 


STRENGTHS AND WEAKNESSES

DESCRIPTION 
STRENGTHS 
WEAKNESSESS 
Ecologic study 
Aggregate data involved (not for specific individuals 
Take advantage of preexisting data, can be used to evaluate programs, polices or regualtions implemted at the ecological level 
Susceptible to confounding exposure and disease or injury outcomes not measured on the same individuals 
Crosssection studies 
All variables measured at a point in time no distinction between potential risk factors and outcomes 
Control over study population and measurements. several associations between variables can be studied at the same time, short time period required 
Potential bias from low response rate, higher proportion of longterm survivors, does not yield incidence or relative risk 
Case study 
A snapshot description of a problem or situation for an individual or group 
indepth description, provides clues to identify a new disease or adverse health effect resulting from exposure or experience 
Conclusion limited to the individual, group,and or context under study, cannot be used to establish a cause effect relationship 
CRUDE RATES VS AGEADJUSTED RATE
Crude rate is calculated without any restrictions such as age. however, these rates are limited if the epidemiologist is trying to compare them between subgroups of the population or over time because of potential confounding influences, such as differences in the age distribution between groups. 
An ageadjusted rate is a weighted average of the agespecific rates.Rates based on data covering age intervals of 5 or 10 years are generally preferred because they are more stable than rates based on singleyear age intervals. 
Indirect method of age adjustment In situations in which agespecific rates are unstable because of small or missing numbers, age adjustment is still possible with the indirect method. 
standard morbidity/mortality ratio (SMR). 
Interpretation ■ SMR = 1: The healthrelated states or events observed were the same as expected from the agespecific rates in the standard population. ■ SMR > 1: More healthrelated states or events were observed than expected from the agespecific rates in the standard population. ■ SMR < 1: Fewer healthrelated states or events were observed than expected from the agespecific rates in the standard population. 


4 TYPES OF DATA
Nominal 
unordered categories or classes (e.g., gender, race/ethnicity, marital status, occupation). 
Ordinal 
additional information provided by the order among categories (e.g., stage or grade of cancer). 
Dsicrete 
integers or counts that differ by fixed amounts, with no intermediate values possible (e.g., number of new cases of lung cancer reported in the United States in a given year, number of children, number of sick days taken in a month). 
Continous 
measurable quantities not restricted to taking on integer values (e.g., age, weight, temperature). 
TABLES, GRAPHS AND NUMERICAL MEASURES
The simplest table is the frequency distribution, which is a complete summary of the frequencies, or number of times each value appears. 
Epidemic Curve is a histogram that shows the course of an epidemic by plotting the number of cases by time of onset. ■ A stemandleaf plot is a display that organizes data to show their distribution. 
A histogram shows a frequency distribution for discrete or continuous data. 
Bar charts are often used for graphically displaying a frequency distribution that involves nominal or ordinal data. 
Numerical Methods
Measures of central tendency refer to ways of designating the center of the data. The most common measures are the arithmetic mean, geometric mean, median, and mode 
Arithmetic and Geometric Mean 
Measures of dispersion, also called the spread or variability, are used to describe how much data values in a frequency distribution vary from each other and from the measures of central tendency. 
Numerical Methods
Measures of central tendency refer to ways of designating the center of the data. The most common measures are the arithmetic mean, geometric mean, median, and mode 
Arithmetic and Geometric Mean 
Measures of dispersion, also called the spread or variability, are used to describe how much data values in a frequency distribution vary from each other and from the measures of central tendency. 

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