Difference and percent difference in Tableau - Cell-by-Cell Formula Trace

Sample Data

Monthly sales data for four months

Cell	Value
A1	Month
B1	Sales
A2	Jan
B2	1000
A3	Feb
B3	1200
A4	Mar
B4	900
A5	Apr
B5	1500

Formula Trace

Difference = SUM([Sales]) - LOOKUP(SUM([Sales]), -1) Percent Difference = (Difference) / LOOKUP(SUM([Sales]), -1)

Step 1: SUM([Sales]) at Feb = 1200

Step 2: LOOKUP(SUM([Sales]), -1) at Feb = SUM([Sales]) at Jan = 1000

Step 3: Difference at Feb = 1200 - 1000

Step 4: Percent Difference at Feb = 200 / 1000

Step 5: SUM([Sales]) at Mar = 900

Step 6: LOOKUP(SUM([Sales]), -1) at Mar = SUM([Sales]) at Feb = 1200

Step 7: Difference at Mar = 900 - 1200

Step 8: Percent Difference at Mar = -300 / 1200

Step 9: SUM([Sales]) at Apr = 1500

Step 10: LOOKUP(SUM([Sales]), -1) at Apr = SUM([Sales]) at Mar = 900

Step 11: Difference at Apr = 1500 - 900

Step 12: Percent Difference at Apr = 600 / 900

Cell Reference Map

     A       B
  +-------+-------+
1 | Month | Sales |
  +-------+-------+
2 | Jan   | 1000  |
  +-------+-------+
3 | Feb   | 1200  | <-- Uses B3 and LOOKUP to B2
  +-------+-------+
4 | Mar   | 900   | <-- Uses B4 and LOOKUP to B3
  +-------+-------+
5 | Apr   | 1500  | <-- Uses B5 and LOOKUP to B4
  +-------+-------+

The formula uses current month sales (column B) and previous month sales via LOOKUP function to calculate difference and percent difference.

Result

     A       B       C           D
  +-------+-------+-----------+--------------+
1 | Month | Sales | Difference| Percent Diff |
  +-------+-------+-----------+--------------+
2 | Jan   | 1000  |           |              |
  +-------+-------+-----------+--------------+
3 | Feb   | 1200  | 200       | 0.20         |
  +-------+-------+-----------+--------------+
4 | Mar   | 900   | -300      | -0.25        |
  +-------+-------+-----------+--------------+
5 | Apr   | 1500  | 600       | 0.6667       |
  +-------+-------+-----------+--------------+

The Difference column shows the change in sales from the previous month. The Percent Diff column shows that change as a decimal fraction of the previous month's sales.

Sheet Trace Quiz - 3 Questions

What is the Difference value for March?

A 300

B -300

C 2100

D 900

Key Result

Difference = Current value minus previous value; Percent Difference = Difference divided by previous value

Practice

1. What does the Difference calculation in Tableau typically show?

easy

A. The total sum of all values

B. The average value over time

C. How much a value changed between two points

D. The maximum value in a dataset

Solution

1. Step 1: Understand the meaning of Difference

   Difference measures the change in value from one point to another, showing increase or decrease.

2. Step 2: Compare with other options

   Sum, average, and maximum describe different calculations unrelated to change between points.

3. Final Answer:

   How much a value changed between two points -> Option C

4. Quick Check:

   Difference = Change amount [OK]

Hint: Difference = New value minus old value [OK]

Common Mistakes:

• Confusing difference with total sum
• Thinking difference shows average
• Mixing difference with maximum value

2. Which of the following Tableau calculations correctly computes the percent difference between the current and previous value?

easy

A. ([Current Value] - [Previous Value]) / [Previous Value]

B. ([Previous Value] - [Current Value]) / [Current Value]

C. [Current Value] + [Previous Value] / [Previous Value]

D. [Current Value] * [Previous Value]

Solution

1. Step 1: Recall percent difference formula

   Percent difference = (New - Old) / Old, showing change relative to original.

2. Step 2: Match formula to options

   ([Current Value] - [Previous Value]) / [Previous Value] matches formula exactly; others either reverse or misuse operations.

3. Final Answer:

   ([Current Value] - [Previous Value]) / [Previous Value] -> Option A

4. Quick Check:

   Percent difference = (New - Old) / Old [OK]

Hint: Percent difference = difference divided by previous value [OK]

Common Mistakes:

• Swapping numerator terms
• Dividing by current value instead of previous
• Using addition or multiplication instead of division

3. Given this Tableau calculated field:
LOOKUP(SUM([Sales]), 0) - LOOKUP(SUM([Sales]), -1)
What does this calculation return?

medium

A. The percent difference in sales between current and previous row

B. The total sales for all rows

C. The sum of sales for the previous row only

D. The difference in sales between the current and previous row

Solution

1. Step 1: Understand LOOKUP function usage

   LOOKUP(SUM([Sales]), 0) returns current row sales; LOOKUP(SUM([Sales]), -1) returns previous row sales.

2. Step 2: Calculate difference

   Subtracting previous from current gives the difference in sales between rows.

3. Final Answer:

   The difference in sales between the current and previous row -> Option D

4. Quick Check:

   LOOKUP difference = sales change [OK]

Hint: LOOKUP with offset -1 gets previous row value [OK]

Common Mistakes:

• Thinking it calculates percent difference
• Assuming it sums all sales
• Confusing current and previous row values

4. You wrote this Tableau formula to calculate percent difference:
(SUM([Sales]) - LOOKUP(SUM([Sales]), -1)) / SUM([Sales])
But the results seem incorrect. What is the likely error?

medium

A. Not using WINDOW_SUM instead of SUM

B. Dividing by current sales instead of previous sales

C. Subtracting current sales from previous sales

D. Using LOOKUP with offset -1 incorrectly

Solution

1. Step 1: Analyze denominator in formula

   The formula divides by SUM([Sales]) which is current sales, but percent difference should divide by previous sales.

2. Step 2: Identify correct denominator

   Correct formula divides by LOOKUP(SUM([Sales]), -1) to get previous sales as denominator.

3. Final Answer:

   Dividing by current sales instead of previous sales -> Option B

4. Quick Check:

   Percent difference denominator = previous value [OK]

Hint: Divide by previous value, not current, for percent difference [OK]

Common Mistakes:

• Using current value as denominator
• Confusing subtraction order
• Misusing LOOKUP offset

5. You want to create a Tableau dashboard showing monthly sales percent difference compared to the previous month. Which approach correctly calculates this and handles months with zero sales without errors?

hard

A. Use (SUM([Sales]) - LOOKUP(SUM([Sales]), -1)) / NULLIF(LOOKUP(SUM([Sales]), -1), 0) to avoid division by zero

B. Use (SUM([Sales]) - LOOKUP(SUM([Sales]), -1)) / SUM([Sales]) ignoring zero sales

C. Use SUM([Sales]) / LOOKUP(SUM([Sales]), -1) without subtraction

D. Use WINDOW_SUM(SUM([Sales])) - WINDOW_SUM(SUM([Sales])) for difference

Solution

1. Step 1: Calculate percent difference with zero check

   Percent difference = (Current - Previous) / Previous; use NULLIF to avoid division by zero errors when previous sales are zero.

2. Step 2: Evaluate options for correctness

   Use (SUM([Sales]) - LOOKUP(SUM([Sales]), -1)) / NULLIF(LOOKUP(SUM([Sales]), -1), 0) to avoid division by zero uses NULLIF to prevent errors; others either divide by current or ignore zero sales causing errors or wrong results.

3. Final Answer:

   Use (SUM([Sales]) - LOOKUP(SUM([Sales]), -1)) / NULLIF(LOOKUP(SUM([Sales]), -1), 0) to avoid division by zero -> Option A

4. Quick Check:

   Use NULLIF to handle zero denominator [OK]

Hint: Use NULLIF to prevent division by zero in percent difference [OK]

Common Mistakes:

• Dividing by current sales instead of previous
• Ignoring zero sales causing errors
• Using sum differences without percent calculation