Descriptive Statistics

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Descriptive Statistics

• 01/31/2006

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Causality

• Dependent Variable
• The variable to be explained (effect)
• Object of the research
• Independent Variable
• The variable expected to account for (the cause
  of) the dependent variable
• Usually occurs earlier in time

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Levels of Measurement

• Nominal
• Data is assigned to one of a set of categories
• Ordinal
• Data is assigned to rank-ordered categories
• Interval-Ratio
• Measurements for all cases are expressed in the
  same units

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Cumulative Property of Levels of Measurement
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Discrete vs. Continuous

• Statistical operations are determined by the type
  of data
• Discrete variables have a minimum unit of
  measurement

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Type of Statistic

• Descriptive
• Help us organize and describe the data
• Inferential
• Concerned with making predictions or inferences
  about a population from observations and analyses
  of a sample

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Measures of Central Tendency

• Numbers that describe what is typical of the
  distribution of data
• Mode
• The category or score with the highest frequency
  in the distribution

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Measures of Central Tendency

• Median
• Can be calculated for variables that are at least
  ordinal
• The score that divides the distribution into two
  equal parts

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Measures of Central Tendency

• Mean
• Best-known and most widely used statistic
• Used with interval level data
• The arithmetic average obtained by adding up al
  the scores and dividing by the total number of
  scores
• Center of gravity

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Distribution of the Data

• Develop a sense of how spread out the data is
• Frequency distribution
• Works best with nominal/ordinal data
• Histogram

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Shape of the Distribution

• Symmetrical
• If the frequencies at the right and left tails of
  the distribution are similar
• Skewed
• There are few extreme values on one side of the
  distribution

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Choosing the Proper Measure

• Level of Measurement
• When the distribution is skewed the mean may give
  misleading information