Epi 3 - Descriptive Statistics Flashcards
Descriptive Statistics
Non-comparative, simple descriptions of various elements of study’s data
Provides readers w/ description of data, study subjects (patients), and elements about them
What do the Measures of Central Tendency describe about data set?
Describe the dispersion or spread and variance + consistency (variance + SD) of all data
Mean, Median, Mode Minimum, Maximum, range Interquartile Range Variance Standard deviation
Which Measures of Central Tendency describe the consistency and variance of data?
variance
standard deviation
Which of the REQUIRED ASSUMPTIONS of interval/ratio data for proper use in Parametric Test can be determine by descriptive statistics?
Normally-distributed
Normal distribution of data can be determined 3 different ways via descriptive statistics
Parametric Test =
All statistical data tests for Interval/Ratio data
3 ways to check for symmetry or normal distribution in data
1) Mean/Median
2) Look at shape of Graphical Representation
3) Skewness value or Kurtosis value; value 0 = normal distribution both
Mean
Average all data
Only significant in interval data
Median
The middle value of data set when in ascending order
Mode
Highest frequency value of data set
Interquartile Range
Middle 50% of data values of normally-distributed data
Between 25th - 75th percentiles
If normal-distribution, the mean/median of data =
~ 1
Mean & median = equal or near equal values
Data distribution is skewed anytime
Median DIFFERS from Mean
mean > median =
Positively Skewed Distribution Data - skew to RIGHT
- Mean LARGER than Median
- Asymmetrical curve = longer ‘tail’ extends/points to right of graph (extends positive values graph)
- Skewness and Kurtosis NOT = 0
mean < median =
Negatively Skewed Distribution Data - skew to LEFT
- Mean SMALLER than Median
- Asymmetrical curve = longer ‘tail’ extends/points to LEFT of graph (toward negative graph values)
- Skewness / Kurtosis NOT = 0
mean = median
Normally-distributed data
Symmetrical bell curve with data even dispersed either side of mean
Skewness
Statistical value that discerns whether or not data equally distributed.
Skewness value = 0 = normally-distributed data
Kurtosis
Measure extent that the data clusters around mean value. Kurtosis statistic = 0 then data Normally-distributed
Fxn of SD: LOWER stand. dev. = MORE data clusters around mean; HIGHER stand. dev. = LESS data clusters around mean - more dispersed
Kurtosis value = 0 = normal distribution
+ value = more cluster around mean
- value = less cluster
Describe relationship between standard deviation & kurtosis
Small standard deviation = more clustering = + kurtosis value
Large standard deviation = less clustering = - kurtosis
Which descriptive statistic is used to describe and compare Continuous (Interval/Ratio data)? Why?
MEAN
B/C: Discrete data (Nominal/Ordinal) do NOT have consistency their scales of measurement / categories - thus - if mean = value not distinctly fall within their scale the value is irrelevant because it has no meaning
- ex.: Nominal data - gender (male = 0, female = 1) and mean = 0.6 – value has no meaning or relevance to scale
Ordinal data - questionnaire responses (never = 0, almost never = 2, sometimes = 3 almost always= 4, always = 5) and mean = 3.6 – value again is irrelevant because values between scale no meaning
What descriptive statistic is useful for describing and comparing Ordinal Data?
MEDIAN
Provides a sense of the central, middle value data that is still relevant Ordinal data set because unlike mean, median value will fall within the scale of measure.
Median - central value when data in ascending order
What percentile of data falls within 1 standard deviation of mean value of a normally distributed data set?
68%
+/- 34% either side mean value
What percentile of data falls within 2 standard deviations of mean value of a normally distributed data set?
95%
+/- 13.5% either side 1 sd;
13.5 + 34 = 47.5% on either side of mean
What percentile of data falls within 3 standard deviations of mean value of a normally distributed data set?
99.7%
+/- 2.35% either side 2 sd
Variance and Standard Deviation descriptive values provide what information about data set?
Variability and Consistency
