Data Handling Ex 3.3 Class 7 NCERT: Bar Graphs

What is a Bar Graph?

A bar graph is a picture that represents data using rectangular bars. Think of it like building towers with blocks, where the height of each tower shows a specific number. In a bar graph, all bars have the same width, but their height (or length) changes depending on the value they represent. This makes it super easy to compare things at a glance. For example, if you make a bar graph of marks in different subjects, you can instantly see your highest and lowest scores. Key parts of a bar graph include the horizontal X-axis (where we usually put the categories, like 'Maths', 'Science', 'English') and the vertical Y-axis (where we mark the numbers or values). Choosing the right 'scale' for the Y-axis is the most important step, as it decides how tall your bars will be.

Step-by-Step Guide to Drawing a Bar Graph

1. Step 1: Collect and Organize Data — First, gather the information you want to show. Let's say we have the number of students who like different fruits: Apples (25), Bananas (30), Oranges (15), Grapes (20).
2. Step 2: Draw the Axes — Draw two perpendicular lines on graph paper. The horizontal line is the X-axis, and the vertical line is the Y-axis.
3. Step 3: Choose a Suitable Scale — Look at your highest value (30). We need a scale that fits this comfortably. Let's choose a scale where 1 unit (one small square on the graph paper) represents 5 students. We will mark 0, 5, 10, 15, 20, 25, 30, 35... on the Y-axis.
4. Step 4: Label the Axes — Label the X-axis 'Fruits' and the Y-axis 'Number of Students'. Mark the fruit names (Apples, Bananas, etc.) on the X-axis at equal distances.
5. Step 5: Draw the Bars — Now, draw the rectangular bars. For Apples (25), draw a bar up to the 25 mark on the Y-axis. For Bananas (30), draw a bar up to the 30 mark. Do this for all fruits. Make sure all bars have the same width and have equal gaps between them.

Comparing Data with Double Bar Graphs

What if you want to compare two sets of data at the same time? For example, comparing the marks of Rohan and Priya in five different subjects. This is where a double bar graph comes in! It's just like a normal bar graph, but for each category (like 'Maths'), you draw two bars side-by-side, one for Rohan and one for Priya. These two bars are stuck together, and then there's a gap before the next pair of bars for the next subject. To avoid confusion, you must use a key or legend. For instance, you could shade Rohan's bars blue and Priya's bars red. The key would show that 'Blue bar = Rohan's marks' and 'Red bar = Priya's marks'. This allows for a direct visual comparison for each subject.

Exam Tip: Choosing the Right Scale

A very common place to lose marks is by choosing a poor scale. If your data values are 50, 100, 80, 120, a scale of '1 unit = 1' would make your graph huge and impossible to draw. A scale of '1 unit = 50' would make the bars too small and hard to read. A good choice here would be '1 unit = 10' or '1 unit = 20'.

Quick Rule: Look at the highest value in your data. Choose a scale that allows this highest value to fit neatly on your page. Also, try to pick a simple number for your scale like 2, 5, 10, 20, 50, or 100 to make marking the points easier.

Practice Questions with Solutions

• Q: The number of shirts sold at a shop from Monday to Friday are: Monday-30, Tuesday-45, Wednesday-20, Thursday-35, Friday-50. Draw a bar graph to represent this data. A: Step 1: Draw the X-axis (for Days) and Y-axis (for Number of Shirts). Step 2: Choose a scale. The highest value is 50. Let's take the scale as 1 unit = 10 shirts. Mark 10, 20, 30, 40, 50 on the Y-axis. Step 3: Mark 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday' on the X-axis at equal intervals. Step 4: Draw a bar of height 3 units for Monday (since 30/10 = 3), 4.5 units for Tuesday, 2 units for Wednesday, 3.5 units for Thursday, and 5 units for Friday. Final answer: A bar graph with 5 bars representing the shirt sales for each day.
• Q: A bar graph shows the production of wheat (in tonnes) in a farm for 5 years. The bar for 2020 is the tallest and the bar for 2018 is the shortest. What information do you get from this? A: Step 1: Identify what the height of the bar represents. The height represents the production of wheat in tonnes. Step 2: Understand what 'tallest' and 'shortest' mean in this context. The tallest bar corresponds to the maximum value, and the shortest bar corresponds to the minimum value. Final answer: The production of wheat was maximum in the year 2020 and minimum in the year 2018.
• Q: The marks (out of 100) obtained by a student in Term 1 and Term 2 are given: Subject: English(67, 70), Hindi(72, 65), Maths(88, 95), Science(81, 85). Represent this data using a double bar graph. A: Step 1: Draw X and Y axes. Choose a scale for the Y-axis (Marks), for instance, 1 unit = 10 marks. Mark 10, 20,...100 on the Y-axis. Step 2: Mark the subjects (English, Hindi, Maths, Science) on the X-axis. Step 3: Create a key. For example, an unshaded bar for Term 1 and a shaded bar for Term 2. Step 4: For English, draw an unshaded bar up to 67 and an adjacent shaded bar up to 70. For Hindi, draw an unshaded bar up to 72 and a shaded bar to 65. Repeat for all subjects. Final answer: A double bar graph with pairs of bars for each subject, showing a comparison of Term 1 and Term 2 marks, along with a key.
• Q: From the double bar graph created in the previous question, in which subject has the child improved their performance the most? A: Step 1: For each subject, find the difference in marks between Term 2 and Term 1. Step 2: English: 70 - 67 = 3 (Improvement). Hindi: 65 - 72 = -7 (Decline). Maths: 95 - 88 = 7 (Improvement). Science: 81 - 85 = 4 (Improvement). Step 3: Compare the improvements. The improvements are 3, 7, and 4. The highest value is 7. Final answer: The child has improved their performance the most in Maths.

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