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Bar Graphs & Histograms
What is the difference between a bar graph and a histogram? There are two differences • The type of data that is presented • The way they are drawn – The difference in the way that bar graphs and histograms are drawn is that the bars in bar graphs are usually separated, in histograms the bars are adjacent to each other. – This is not always true however. Sometimes you see bar graphs with no spaces between the bars but histograms are never drawn with spaces between the bars
Bar Graphs Bar graphs are usually used to display "categorical data", that is data that fits into categories. For example suppose that I offered to buy donuts for six people and three said they wanted chocolate covered, 2 said plain and one said with icing & sprinkles.
Histograms are usually used to present "continuous data", that is data that represents measured quantity where, at least in theory, the numbers can take on any value in a certain range. A good example is weight. If you measure the weights of a group of adults you might get and numbers between 90 lbs. and 240 lbs. We usually report our weight as pounds or to the nearest 1/2 lb. but we might do so to the nearest 1/10 lb. depending on how accurate the scale is.
To make a histogram… • Remember: A histogram is a particular kind of bar graph. • The data that is being represented is in intervals… • So…to organize your data, make a • Frequency table first! • Then transfer the organized data to the histogram
Histogram example: The weight data would then be collected into categories to present a histogram. Weights of adults For Example: 40 # of people 30 20 10 90 I 150 I Pounds 210 I might be a histogram for weights (with the appropriate scale on the vertical axis). Here the data has been collected into categories of 30 lbs.
To sum it all up… • A histogram is a particular kind of bar graph. • If you make a bar graph of the favorite colors of children in Mrs. Flaherty's 4 th grade class (6 for pink, 8 for blue, 1 for black, etc. ) that doesn't qualify as a histogram, because the colors don't correspond to any numerical values, and their order is arbitrary. • A different kind of example: you could make a bar graph of monthly returns on your stock investment in January, February, etc. Here, there IS a natural order to the bars, but it still doesn't qualify as a histogram.
To finish summing it all up… • A histogram is a bar graph of frequencies of different numerical values within a population. The most straightforward kind of example may be a bar graph of the number of 4 th grade students by height: how many between 4' and 4'1" between 4'1" and 4'2" between 4'2" and 4'3" • Both axis use numbers!