Knowledge Quiz: Tutorial 2

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geom_bar(aes(x = class)) produces a bar chart without you ever specifying a y mapping. Where does the height of each bar come from?

ggplot2 uses the alphabetical rank of each category as its height.ggplot2 automatically counts the number of rows for each level of x and maps that count to y.geom_bar() requires a y mapping and this code would fail.The plot has no meaningful height; bars are all drawn the same size.

• Wrong: alphabetical rank plays no role in bar height.
• Correct: geom_bar()’s default statistical transformation, stat_count(), counts observations per x level.
• Wrong: geom_bar() works perfectly well with only an x mapping; y is optional because of the default count statistic.
• Wrong: bar heights do vary and are meaningful, they represent counts.

Which of the following statements about histograms and density plots are TRUE? (Select all that apply.)

Choosing a wider binwidth in a histogram can hide outliers that a narrower binwidth would reveal.Density plots and histograms always show identical distribution shapes, regardless of settings.A histogram’s bar heights represent observation counts (given a chosen bin width).A density plot’s y-axis is a probability density, and the total area under the curve is 1.

• TRUE: wider bins smooth over fine detail, which can obscure isolated extreme values.
• FALSE: histogram shape depends on binwidth, and density plot shape depends on smoothing/bandwidth settings, the two do not automatically match.
• TRUE: each bar counts the observations falling into that bin.
• TRUE: this is the defining property of a probability density.

In a geom_boxplot(), what does the thick line inside the box represent?

The mean of the data.The mode of the data.The median of the data.The maximum value in the data.

• Wrong: the mean is not directly marked on a standard boxplot.
• Wrong: boxplots do not depict the mode directly.
• Correct: the thick middle line marks the 50th percentile (median).
• Wrong: the maximum (excluding outliers) is marked by the end of the upper whisker, not the middle line.