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Stat 350, Lecture # 2

Stat 350, Lecture # 2. We have talked about graphical displays of variables. Categorical variables: Bar Chart and Pie chart. Quantitative Variables: Histogram and to talk about center, spread and shape. We have also taken a quick glance at outliers.

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Stat 350, Lecture # 2

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  1. Stat 350, Lecture # 2 • We have talked about graphical displays of variables. • Categorical variables: Bar Chart and Pie chart. • Quantitative Variables: Histogram and to talk about center, spread and shape. • We have also taken a quick glance at outliers.

  2. More graphical displays for quantitative variables: • Stem and leaf plot: Quicker way for small data sets. Presents more detailed information and uses each individual observation. • Step1. Each observation has a leaf which is the final or rightmost digit . The remaining digits constitute the stem.

  3. For example if you have an observation of 345, 5 is the leaf and 34 is the stem. • Step 2: Write the stems in a vertical column with smallest at the top and draw a vertical line at the right of the column. • Step 3: Write ecah leaf in the row to the right of the stem in increasing order out from the stem.

  4. Stem and leaf for Automobile fuel economy: • Data: 25,26,29,24,26,29,30,28,28,29,27,28, 23,23,25,26,33,29,26,24,28,26,16,25,30 Minimum: 16 Maximum 33 1 6 2 3 3 4 4 5 5 5 6 6 6 6 6 7 8 8 8 9 9 3 0 0

  5. Time Plots (for variables measured over time) • Many variables are measured over time. E.g., Height of a child, price of a stock at the end of each day, amount of rainfall each year, unemployment rate each month.. • A time plot plots each observation against time at which it was measured. time is always on the horizontal scale and points are often connected by line.

  6. Example: Table 1.6 (Deaths from cancer over the years)

  7. Summary • Individuals and variables • Categorical and quantitative • Talk about variables through graphical displays. • Categorical Variable: Bar chart and pie chart • Quantitative: Histogram and stemplots • Variables recorded over time: Time plot

  8. Summary • Look for overall pattern and huge deviations from it. • Look at shape (symmetric, skewed) • Try to have an idea about center and spread • Look into outliers • In time plots look for trends.

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