1. Introduction to Statistics
Do I have to??

2. Why we “do it”
"What we really want to get at [in health care research] is not how many reports have been done, but how many people's lives are being bettered by what has been accomplished. In other words, is it being used, is it being followed, is it actually being given to patients—... What effect is it having on people—"
Rep. John Porter (R-IL), retired chairman House Appropriations Subcommittee on Labor, Health and Human Services (HHS), and Education

3. Is Statistics Important?
Statistics is important because we can use it to find out whether something we observe can be applied to new and different situations.
Knowing this allows us to plan for the future, and to make decisions about how to allocate our scarce resources of money, energy, and ultimately life.
We use the term generalizable: can what we know help to predict what will happen in new and different situations?

4. Why Statistics
Scientific knowledge represents the best understanding that has been produced by means of current evidence.
Research design, if used properly, strengthens the objectivity of the research.
Statistical methods allow us to compare what is actually observed to what is logically expected.

5. Why Statistics (cont’d)
Knowledge of statistics . . .
Useful in conducting investigations
Helpful the preparing and evaluating research proposals.
Vital in deciding whether claims of a researcher are valid
Keep abreast of current developments.
Effective presentations of the findings.

7. Evils of Pickle Eating
Pickles are associated with all the major diseases of the body. Eating them breeds war and Communism. They can be related to most airline tragedies. Auto accidents are caused by pickles. There exists a positive relationship between crime waves and consumption of this fruit of the cucurbit family. For example

8. Evils of Pickle Eating (cont’d)
Nearly all sick people have eaten pickles. 99.9% of all people who die from cancer have eaten pickles.
100% of all soldiers have eaten pickles.
96.8% of all Communist sympathizers have eaten pickles.
99.7% of the people involved in air and auto accidents ate pickles within 14 days preceding the accident.
93.1% of juvenile delinquents come from homes where pickles are served frequently. Evidence points to the long-term effects of pickle eating.
Of the people born in 1839 who later dined on pickles, there has been a 100% mortality.

9. Evils of Pickle Eating (cont’d)
All pickle eaters born between 1849 and 1859 have wrinkled skin, have lost most of their teeth, have brittle bones and failing eyesight-if the ills of pickle eating have not already caused their death.
Even more convincing is the report of a noted team of medical spe­cialists: rats force-fed with 20 pounds of pickles per day for 30 days de­veloped bulging abdomens. Their appetites for WHOLESOME FOOD were destroyed.

10. Evils of Pickle Eating (cont’d)
In spite of all the evidence, pickle growers and packers continue to spread their evil. More than 120,000 acres of fertile U.S. soil are devoted to growing pickles. Our per capita consumption is nearly four pounds.
Eat orchid petal soup. Practically no one has as many problems from eating orchid petal soup as they do with eating pickles.
EVERETT D. EDINGTON

13. Types of Statistics Descriptive Statistics Examples
enumerate, organize, summarize, and categorize
graphical representation of data.
these type of statistics describes the data.
Examples
means and frequency of outcomes
charts and graphs

14. Types of Statistics Inferential Statistics Examples
drawing conclusions from incomplete information.
they make predictions about a larger population given a smaller sample
these are thought of as the statistical test
Examples
t-test, chi square test, ANOVA, regression

16. Creighton University Medical Center
Variables
J.D. Bramble, Ph.D.
Creighton University Medical Center
Med Fall 2006

17. Types of Data Qualitative Quantitative
data fall into separate classes with no numerical relationship
sex, mortality, correct/incorrect, etc.
Quantitative
numerical data that is continuous
pharmaceutical costs, LOS, etc.

18. Parameters and Statistics
characteristics of the population
calculating the exact population parameter is often impractical or impossible
Statistics
characteristics of the sample
represent summary measures of observed values

19. Types of Variables
Variables are symbols to which numerals or values are assigned
e.g. X and Y are variables
Dependent (Y’s), that which is predicted
Independent (X’s), that which predicts
Extraneous (Confounding or Control)
statistical models “adjust” for their influence

20. Independent variables
Independent variables are the presumed cause of the the dependent variable
The variable responsible for the change in the phenomena being observed
Nothing is for sure, so avoid the word ‘cause’ and think in terms of independent and dependent variables

21. Dependent variables Also referred to as the outcome variable
The outcome of the changes due to the independent variables
Example: y = a + bx

22. Confounding variables
Additional variables that may effect the changes in the dependent variable attributed to the independent variables.
These variables are controlled by measuring them and statistical methods adjust for there influence.
Sometimes referred to as control variables

23. Active vs. attribute variables
Active variables are those variables under the control of the researcher
controlled experimental studies
e.g., amount of drug administered
Attribute variables can not be manipulated by the researcher
quasi-experimental studies
e.g.,sex or age of subject; blood pressure; smoker

24. The Wrong data Leads to Migraines

25. Levels of Measurement Categorical Variables Continuous Variables
Nominal Scale
Ordinal Scale
Continuous Variables
Interval Scale
Ratio Scale

26. Continuous Variables
Continuous variables are measured and can take on any value along the scale
quantitative variables
measured on a interval or ratio level
Examples
Age, income, number of medications

27. Categorical Variables
Categorical variables are measured as dichotomous or polytomous measures
qualitative variables
measured on a nominal or ordinal level
Examples
sex; smoking status; ownership
Categorizing continuous variables

28. Nominal measurement scale
Used for qualitative data
Two or more levels of measurement
The name of the groups does not matter
Examples
Sex (Male/Female)
Smoker (Yes/No)
Political Party (Rep, Dem, Ind)

29. Ordinal measurement scale
All the properties of nominal plus . . .
The groups are ordered or ranked
Intervals between groups are not necessarily equal
Example:
Income (low, med, high)
Disease severity
Likert scales

30. Interval measurement scale
All properties of nominal and ordinal plus . . .
A scale is used to measure the response of the study subjects
The intervals scale’s units are equal; however arbitrary (e.g., a relative scale)
Examples:
Temperature on Fahrenheit scale

31. Ratio measurement scale
All properties of the previous scales plus . . .
An absolute zero point
Can perform mathematical operations
Highest level of measurement
Examples
Income, age, height, weight

32. Measures of Central Tendency and Variation
Summarizing Data
Measures of Central Tendency and Variation
The mean is our usual concept of an overall average - add up the items and divide them by the number of sharers (100 candy bars collected for five kids next Halloween will yield 20 for each in a just world). The median, a different measure of central tendency, is the half-way point. If I line up five kids by height, the median child is shorter than two and taller than the other two (who might have trouble getting their mean share of the candy). A politician in power might say with pride, "The mean income of our citizens is $15,000 per year." The leader of the opposition might retort, "But half our citizens make less than $10,000 per year." Both are right, but neither cites a statistic with impassive objectivity. The first invokes a mean, the second a median.

33. Mean Arithmetic mean the balance point sum all observations
divide the sum by the number of observations
Means are higher than medians in such cases because one millionaire may outweigh hundreds of poor people in setting a mean; but he can balance only one mendicant in calculating a median

34. Median Divides the distribution into two equal parts.
Considered the most “typical” observation
Less sensitive to extreme values

35. Calculating Medians To find the median value: q(n+1)
41, 28, 34, 36, 26, 44, 39, 32, 40, 35, 36, 33
order data in ascending order
26, 28, 32, 33, 34, 35, 36, 36, 39, 40, 41, 44
Apply the median location formula: 0.5(12+1) = 6.5
Note: this is ONLY the location of the median

36. Quantiles
Quantiles are those values that divide the distribution into n equal parts so that there is a given proportion of data below each quantile.
The median is the middle quantile.
Quartiles are also very common (25, 50, 75)
If we divided the distribution into 100 then we have percentiles.

37. Mode The observation that occurs most frequently
Graphically it is the value of the peak of the distribution.
Frequency often may be bimodal--two modes.
If values are all the same--no mode exists

38. Single Modal

39. Bimodal Example

40. Symmetrical: The relationship between the Mean, Median, & Mode

41. Positive Skew: The relationship between the Mean, Median, & Mode

42. Negative Skew: The relationship Between the Mean, Median, & Mode

43. Summarizing Data Frequency distributions Measures of central tendency
The tendency of data to center around certain numerical and ordinal values.
Three common measures:
mean, median, & mode
Measures of variation
standard deviation

44. Five Figure Summary Median Quartiles Maximum Minimum
Can be shown in a box and whisker plot

45. Which Measure? Mean Median Mode numerical data symmetric distribution
ordinal data
skewed distribution
Mode
bimodal distribution
most popular

46. Variation Must also report measures of variation
Measures of variability reflect the degree to which data differ from one another as well as the mean.
Together the mean and variability help describe the characteristics of the data and shows how the distributions vary from one another.

47. Example of Variation
Take the following three sets of data:          1) 10, 8, 5, 5, 2;         2) 5, 6, 6, 7, 6;           3) 6, 6, 6, 6, 6
In all three cases the mean is 6,
the variability is a lot of variability in set 1
No variability in set 3.
We will discuss three measures of variability: 1) the range; 2) the standard deviation; and 3) variance

48. Measures of Variation Range
the value between the highest and the lowest observations
Range = xmax - xmin
limited usefulness since it only accounts for the extreme values
can also report the inter-quartile range (q3 – q1)

49. Standard Deviation most widely used & preferred measure of variation.
represented by the symbol s or sd
the square root of the variance (s2)
larger values = more heterogeneous distribution
75% of the observations lie between x-2s and x+2s
if the distribution is normal (bell shaped)
67% =
95% =
99.7% =

50. Variance and Std Deviation
Standard Deviation

51. Example
Using data on the sexual activity of male and female subjects can be found in Chatterjee, Handcock, and Simonoff (1995) A casebook for a first course in statistics. New York: Wiley. They provide data on the reported number of sexual partners for 1682 females and 1850 males. The dependent variable is the number of reported partners.

52. Descriptive Statistics
Male Female (n=1850) (n=1685)
Mean
Median
Mode

53. Using Excel When Syntax in Known
Write them right into the spreadsheet
Be sure to start with an equal sign
Use your mouse to highlight data to analyze

54. Using Excel When Syntax in Unknown
Use the wizard and follow in instructions.
All wizards work about the same way.
Select the fx button to select appropriate test
Select category and then desired test

55. Follow the Wizard Either highlight the array or just write it in
These icons reduce/enlarge the Wizard box