Descriptive Statistics – Graphic Guidelines Pie charts – qualitative variables, nominal data, eg. ‘religion’ Bar charts – qualitative or quantitative variables, nominal or interval data, eg. ‘religion’ or ‘margin debt’; time series or cross sectional data Line graphs – quantitative variables, interval data, eg. margin debt; time series data Histograms – quantitative variables, interval data, eg. golf scores; cross sectional data – depicts the SHAPE of a frequency distribution Stem and Leaf Plot– quick and dirty histogram Ogive – depicts a cumulative relative frequency distribution Scattergram – two quantitative variables, eg. Margin vs, the market value

Graphic Deception – some widely used methods Graphs without a scale on one axis Captions or titles intended to influence Reporting only absolute changes in value and not percentage changes Changing the scale of the vertical axis with breaks or truncations Changing the scale of the horizontal axis Changing the width as well as the height of bars or pictogram figures

Summary: data types and graphic techniques Numeric data Nominal data Cross-sectional data Histograms Pie charts Relative frequency histograms Bar charts Ogives Stem and leaf plots Box plots Time-series data Line charts Complex pie, Bar charts or bar charts

Describing the frequency distribution for interval, cross- sectional data Shape Center Spread

Describing distributions SHAPE Graphs Histograms Relative frequency histograms Ogives, Stem and leaf plots Box plots Words Symmetric, skewed, bell shaped, flat, peaked

Describing distributions CENTER Numeric measures Mean Median Mode Mid-point of the range

Descriptive Statistics – Numeric Measures – cont’d. SPREAD (dispersion) Range Symmetric distributions Standard deviation Variance Skewed distributions Quartiles Min Max Interquartile range Percentiles

Z Scores and t-scores Measures distance from the mean in standard deviations Eg. T score for bone density – 1 to 2.5 standard deviations below the norm (mean) for a 23 year old indicates osteopenia; 2.5 or more indicates osteoporosis (X-    z score (X – Xbar)/s = t score

Empirical Rule For mound shaped distributions About 68% of observations are within one standard deviation of the mean About 95% of observations are within two standard deviations of the mean Almost all (99.7%) observations are within three standard deviations of the mean

Sampling ‘Scientific sampling’ is random sampling Simple random samples Stratified random samples Random cluster samples What? Why? How?

What is random sampling? Simple random sample -Every sample with the same number of observations has the same probability of being chosen Stratified random sample – Choose simple random samples from the mutually exclusive strata of a population Cluster sample – Choose a simple random sample of groups or clusters

Why sample randomly? To make valid statistical inferences to a population Conclusions from a convenience sample can be questioned Conclusions from a self-selected sample are SLOP

How can samples be randomly chosen? Random number generators (software) Ping pong balls in a hopper Other mechanical devices Random number tables Slips of paper in a ‘hat’ With or without replacement