Class 7 · Unit 4: Data visualisation · Lesson 4.3 · 40 min

Preparing data

The Chillai Kalan log is a mess: a day written twice, a day left blank, "lots" instead of a number, dates all jumbled. You can't chart that — yet.

Today you will: Clean a messy log in four steps · Mark missing data — never make it up · Find which month was wetter

Story

During Chillai Kalan, the forty coldest days of winter, Bilal's school kept a daily rainfall-and-snow log. Different students filled it in each day.

When Bilal opens it in March, it is a mess: - 3 January is written twice. - 10 January is empty. That was a snow holiday and nobody came to school. - One day says "lots" instead of a number. - The dates are jumbled.

Hiba: “We can't make a chart from this. First we have to prepare it.”

Watch

Four steps to prepare data — — scan code 7.4.3 in the printed book.

Warm up your fingers

8 minutes on typing.com: sentences with numbers.

Term 2 lessons, then type these dates exactly: 2026-01-03, 2026-01-10, 2026-01-17, 2026-01-24.
Use the number row, not the number pad, so both hands stay near home row.

On the laptop

You need: LibreOffice Calc · unit-4/rain-log-messy.ods

Mission 1: Errors and duplicates (pairs, 6 min)

☐1
Open rain-log-messy.ods and save a copy at once as rain-log-clean.ods. Always keep the original.
☐2
Predict: which month was wetter — December or January? Note it.
☐3
Find lots in Rain/Snow (mm). You can't know the real number — delete the word, leave the cell empty.
☐4
Select the data, Data → More Filters → Standard Filter: Date = Not Empty, tick Options → No duplications, Copy results to F1. Use the new copy.

Mission 2: Missing values and order (pairs, 5 min)

☐5
Type missing in Note beside each empty cell. For 10 January only, your teacher gives the weather-station value — type it and note from station.
☐6
Data → Sort by Date, ascending.

Mission 3: Group and answer (pairs, 5 min)

☐7
Add a Month column (Dec or Jan). Under the data: December total and January total with SUM.
☐8
One sentence: which month was wetter, and does a missing value make you less sure? Save.

No laptop today?

Your teacher writes the 12-row messy log on the board. In pairs, rewrite it cleanly: cross out the duplicate, mark the missing values, put the dates in order, and total each month.

Now you know

Prepare before you chart. Messy data makes misleading charts.
Step 1 — errors and duplicates: nothing counted twice or read wrong.
Step 2 — missing values: mark them, or fill from a trusted source and say so. Never invent a number.
Step 3 — order: by date, size or name.
Step 4 — group: by week, month or type, so patterns appear.

Debate it

For 10 January, one student suggests: "Just type 0. Nobody came to school, so nothing fell." Is that honest? What might a weather AI learn from it?

Check yourself — practice, not a test

1. Put CBSE's steps for preparing data in order:
- 1. Arrange data in order
- 2. Fill missing values
- 3. Group related items
- 4. Remove errors or duplicates
2. A value is missing and you don't know it. The most honest thing is to:
- ○ Type 0
- ○ Type the biggest number in the column
- ○ Mark it as missing, or fill it from a trusted source and note that
- ○ Delete the whole dataset
3. If the same day appears twice, its rain will be counted twice in the total.
- ○ True
- ○ False
4. Before you start cleaning, always save a ______ of the original file.

Remember

Clean, fill honestly, sort, group. Then chart.

Story

During Chillai Kalan, the forty coldest days of winter, Bilal's school kept a daily rainfall-and-snow log. Different students filled it in each day.

Hiba: “We can't make a chart from this. First we have to prepare it.”

On the laptop

You need: LibreOffice Calc · unit-4/rain-log-messy.ods

Mission 1: Errors and duplicates (pairs, 6 min)

☐1
Open rain-log-messy.ods and save a copy at once as rain-log-clean.ods. Always keep the original.
☐2
Predict: which month was wetter — December or January? Note it.
☐3
Find lots in Rain/Snow (mm). You can't know the real number — delete the word, leave the cell empty.
☐4
Select the data, Data → More Filters → Standard Filter: Date = Not Empty, tick Options → No duplications, Copy results to F1. Use the new copy.

Mission 2: Missing values and order (pairs, 5 min)

☐5
Type missing in Note beside each empty cell. For 10 January only, your teacher gives the weather-station value — type it and note from station.
☐6
Data → Sort by Date, ascending.

Mission 3: Group and answer (pairs, 5 min)

☐7
Add a Month column (Dec or Jan). Under the data: December total and January total with SUM.
☐8
One sentence: which month was wetter, and does a missing value make you less sure? Save.

No laptop today?

Your teacher writes the 12-row messy log on the board. In pairs, rewrite it cleanly: cross out the duplicate, mark the missing values, put the dates in order, and total each month.

Now you know

Prepare before you chart. Messy data makes misleading charts.

Step 1 — errors and duplicates: nothing counted twice or read wrong.

Step 2 — missing values: mark them, or fill from a trusted source and say so. Never invent a number.

Step 3 — order: by date, size or name.

Step 4 — group: by week, month or type, so patterns appear.

Check yourself — practice, not a test

1. Put CBSE's steps for preparing data in order:

1. Arrange data in order
2. Fill missing values
3. Group related items
4. Remove errors or duplicates

2. A value is missing and you don't know it. The most honest thing is to:

○ Type 0
○ Type the biggest number in the column
○ Mark it as missing, or fill it from a trusted source and note that
○ Delete the whole dataset

3. If the same day appears twice, its rain will be counted twice in the total.

○ True
○ False

4. Before you start cleaning, always save a ______ of the original file.