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I am currently reading the book "Introduction to statistical learning" and on Page 3 there is a problem statement regarding the SMarket data (stock exchange data) fig 2.2 whose left panel states the relation shown is the percentage vs up/down trend of stock market. But here Author is saying that one box for 648 days and one for 602 days.

I don't quite get that. There is no information given for the days. Is the print faulty or am I missing something here? If someone has gone through the book and has any idea, please help me to understand the following chart.

picture of the book page in question

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    $\begingroup$ perhaps he means the number of data points represented in each box plot? $\endgroup$ Aug 28, 2017 at 5:15

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The left-handed panel of Figure 1.2 displays two boxplots of the previous day's percentage changes in the stock index: one for the 648 days for with the market increased on subsequent days, and one for the 602 days for which the market decreased.

The blue boxplot of the left-most figure is based on data from 648 days. These days are not randomly chosen, they are selected because they have a special characteristic: the day after the market increased. The red boxblot in the left-most figure is based on 602 days for which the market decreased the day after.

In short, the red and blue boxplot visualize multiple datapoints.

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The leftmost Boxplot says: There has been 602 days(data points) for which the market value on the previous day decreased. For those observations, we measure the percentage change in market value on the subsequent day, and we plot it (Blue boxplot). As you can see, although for all such observations the market change on the day before was negative, on the subsequent day, their average percentage return was positive (box plot mean is above zero).

The analogous logic is applied for the 643 days(data points) for which the market value on the previous day increased (Red Boxplot).

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