How to Find Fisher’s Index Number using Python

Tuhin Paul
Oct 06
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Article

Introduction
What Is Fisher’s Index Number?
Why Fisher’s Index Matters in Real Life
Step-by-Step Calculation Method
Complete Python Implementation with Test Cases
Best Practices and Common Pitfalls
Conclusion

Introduction

In economics and data analysis, measuring how prices or quantities change over time is essential for informed decision-making. One of the most accurate and widely respected tools for this purpose is Fisher’s Ideal Index Number. Unlike simpler indices, Fisher’s method elegantly balances the biases of two classical approaches—Laspeyres and Paasche—making it the “ideal” choice for real-world applications.

This article walks you through Fisher’s Index Number with a compelling real-world example from e-commerce pricing strategy, provides a clean, error-free Python implementation, and shares practical tips to avoid common mistakes.

What Is Fisher’s Index Number?

Fisher’s Index Number is a geometric mean of the Laspeyres and Paasche index numbers:

Where

Laspeyres Index uses base-year quantities as weights:

Paasche Index uses current-year quantities as weights:

Here

p0,q0 = prices and quantities in the base year
p1,q1 = prices and quantities in the current year

Fisher’s index satisfies both the time reversal and factor reversal tests, making it statistically robust.

Why Fisher’s Index Matters in Real Life

Imagine you're a data scientist at a global e-commerce platform like Amazon or Flipkart. Your team wants to understand how the cost of a standard “tech bundle” (e.g., laptop, mouse, headphones) has changed from 2022 to 2024—not just in price, but in real consumer value.

Consumers don’t buy the same quantities every year. In 2022, they might have bought more budget laptops; in 2024, premium models dominate. Using only base-year quantities (Laspeyres) would overstate inflation, while using only current-year quantities (Paasche) might understate it.

Fisher’s Index gives you a balanced, fair measure—critical for:

Adjusting subscription pricing
Evaluating vendor cost trends
Informing dynamic pricing algorithms

Step-by-Step Calculation Method

Collect data: Prices and quantities for each item in the base year (2022) and the current year (2024).
Compute the Laspeyres Index: Weight current prices by base-year quantities.
Compute Paasche Index: Weight current prices by current-year quantities.
Take geometric mean: Multiply both indices and take the square root.
Multiply by 100 to express as a percentage.

Complete Python Implementation with Test Cases

from typing import List
import math
import unittest

def fishers_index_number(
    base_prices: List[float],
    base_quantities: List[float],
    current_prices: List[float],
    current_quantities: List[float]
) -> float:
    """
    Calculate Fisher's Ideal Index Number.
    
    Parameters:
    - base_prices: Prices in base year
    - base_quantities: Quantities in base year
    - current_prices: Prices in current year
    - current_quantities: Quantities in current year
    
    Returns:
    - Fisher's Index as a percentage (e.g., 112.5 means 12.5% increase)
    
    Raises:
    - ValueError if input lists have mismatched lengths or non-positive values
    """
    if not (len(base_prices) == len(base_quantities) == 
            len(current_prices) == len(current_quantities)):
        raise ValueError("All input lists must have the same length.")
    
    if any(x <= 0 for x in base_prices + base_quantities + 
           current_prices + current_quantities):
        raise ValueError("All prices and quantities must be positive.")

    # Laspeyres Index: (Σ p1*q0) / (Σ p0*q0)
    numerator_l = sum(cp * bq for cp, bq in zip(current_prices, base_quantities))
    denominator_l = sum(bp * bq for bp, bq in zip(base_prices, base_quantities))
    laspeyres = (numerator_l / denominator_l) * 100

    # Paasche Index: (Σ p1*q1) / (Σ p0*q1)
    numerator_p = sum(cp * cq for cp, cq in zip(current_prices, current_quantities))
    denominator_p = sum(bp * cq for bp, cq in zip(base_prices, current_quantities))
    paasche = (numerator_p / denominator_p) * 100

    # Fisher's Index = sqrt(Laspeyres * Paasche)
    fisher = math.sqrt(laspeyres * paasche)
    return round(fisher, 2)


class TestFisherIndex(unittest.TestCase):
    def test_standard_case(self):
        # Tech bundle: [Laptop, Mouse, Headphones]
        base_prices      = [800, 20, 50]      # 2022 prices
        base_quantities  = [100, 300, 200]    # 2022 sales
        current_prices   = [850, 25, 60]      # 2024 prices
        current_quantities = [120, 250, 220]  # 2024 sales
        
        result = fishers_index_number(
            base_prices, base_quantities,
            current_prices, current_quantities
        )
        # Corrected Expected: The calculated index is 108.65, corrected from 108.30.
        self.assertAlmostEqual(result, 108.65, places=2)

    def test_equal_prices_quantities(self):
        # No change → index should be 100
        prices = [10, 20]
        quantities = [5, 10]
        result = fishers_index_number(prices, quantities, prices, quantities)
        self.assertEqual(result, 100.0)

    def test_mismatched_lengths(self):
        with self.assertRaises(ValueError):
            fishers_index_number([1, 2], [1], [1, 2], [1, 2])

    def test_non_positive_values(self):
        with self.assertRaises(ValueError):
            fishers_index_number([1, -2], [1, 2], [1, 2], [1, 2])


if __name__ == "__main__":
    # Example: E-commerce Tech Bundle Analysis
    print("=== Fisher's Index: E-Commerce Pricing Insight ===\n")
    
    base_prices = [800, 20, 50]        # 2022
    base_quantities = [100, 300, 200]
    current_prices = [850, 25, 60]     # 2024
    current_quantities = [120, 250, 220]
    
    index = fishers_index_number(
        base_prices, base_quantities,
        current_prices, current_quantities
    )
    
    print(f"Base Year (2022) Total Expenditure: ${sum(p*q for p,q in zip(base_prices, base_quantities)):,.2f}")
    print(f"Current Year (2024) Total Expenditure: ${sum(p*q for p,q in zip(current_prices, current_quantities)):,.2f}")
    print(f"\nFisher's Ideal Index Number: {index}")
    print(f"Interpretation: Overall price level increased by {index - 100:.2f}%")
    
    # Run tests
    print("\n=== Running Unit Tests ===")
    unittest.main(argv=[''], exit=False, verbosity=2)

Output:

=== Fisher's Index: E-Commerce Pricing Insight ===

Base Year (2022) Total Expenditure: $96,000.00
Current Year (2024) Total Expenditure: $121,450.00

Fisher's Ideal Index Number: 108.65
Interpretation: Overall price level increased by 8.65%

=== Running Unit Tests ===

test_equal_prices_quantities (__main__.TestFisherIndex.test_equal_prices_quantities) ... ok
test_mismatched_lengths (__main__.TestFisherIndex.test_mismatched_lengths) ... ok
test_non_positive_values (__main__.TestFisherIndex.test_non_positive_values) ... ok
test_standard_case (__main__.TestFisherIndex.test_standard_case) ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.000s

OK

Best Practices and Common Pitfalls

Validate inputs: Ensure all prices and quantities are positive and lists are aligned.
Use consistent units: Don’t mix USD and EUR, or units and dozens.
Avoid zero quantities: They distort Paasche/Laspeyres calculations.
Round only at the end: Preserve precision during intermediate steps.
Document your base year: Fisher’s index is meaningless without context.

Conclusion

Fisher’s Index Number isn’t just a textbook formula—it’s a powerful tool for real-world economic insight, especially in dynamic domains like e-commerce, retail, and supply chain analytics. By combining the strengths of the Laspeyres and Paasche indices, it delivers a balanced, statistically sound measure of price change. With the clean, tested Python implementation above, you can integrate Fisher’s Index into dashboards, pricing engines, or inflation-adjustment models—ensuring your decisions are grounded in accurate, fair data.