1. Concept Explanation (Exam-Focused)
Population Forecasting is the process of estimating
the future population of a town or city for designing water supply,
sewerage, drainage, and other public utilities.
📌 Why it is important
in JE Exams:
Most environmental engineering designs (water demand, sewage flow) are based on
future population, not present population.
How questions are framed in exams
✔ Direct numerical using a
specific method
✔ Asking most suitable method
✔ Statement-based questions on assumptions
✔ Comparison between methods
⚠ Golden Rule for Exams:
Method selection is as important as calculation.
2. Important Formulas (Highly Repeated)
1️⃣ Arithmetic Increase Method
Assumes constant increase per decade.
Where:
= Population after n
decades
= Present population
= Average increase per
decade
📌
Used for: Old, stable cities
2️⃣ Geometric Increase Method
(Most Asked)
Assumes constant percentage growth.
Where:
= Growth rate per decade
(decimal)
📌
Used for: Rapidly growing cities
3️⃣ Incremental Increase Method
Considers increase of increase.
📌 Used for:
Moderately growing towns
4️⃣ Logistic Curve Method
Used when growth approaches saturation (rare
numericals in JE).
🧠 Mnemonic Trick:
“A-G-I-L = Arithmetic → Geometric → Incremental → Logistic”
3. Short Notes / One-Liners (Last-Day Revision)
✔ Arithmetic method gives minimum
population
✔ Geometric method gives maximum population
✔ Incremental method is most realistic
✔ Logistic method assumes limiting population
✔ Population is usually forecasted for 30–50 years
4. PYQ Patterns (SSC JE / RRB JE)
🔹 Geometric method
appears most frequently
🔹
MCQs on method suitability
🔹
Traps in percentage → decimal conversion
🔹
Forgetting to divide by number of decades
⚠ Typical Trap:
Growth rate = 20% → r = 0.20 (not 20)
5. Solved Examples
Example 1 (Arithmetic Increase – SSC JE Level)
Population data (in thousands):
|
Year |
Population |
|
1990 |
40 |
|
2000 |
44 |
|
2010 |
49 |
Average increase
= (4 + 5)/2 = 4.5 thousand
Find population in 2030 (n = 2 decades):
✅ Answer: 58,000
Example 2 (Geometric Increase)
Present population = 50,000
Growth rate = 20% per decade
n = 2
6. MCQs (Exam Level – 30 Questions)
1. Population forecasting is required for design of:
A) Structures
B) Water supply
C) Roads
D) Buildings
✅
Ans: B
2. Which method gives maximum population?
A) Arithmetic
B) Incremental
C) Geometric
D) Logistic
✅
Ans: C
3. Arithmetic increase method assumes:
A) Constant % growth
B) Constant increase
C) Decreasing growth
D) Random growth
✅
Ans: B
4. Best method for a rapidly growing city is:
A) Arithmetic
B) Geometric
C) Logistic
D) Graphical
✅
Ans: B
5. Incremental increase method considers:
A) Only increase
B) Only percentage
C) Increase of increase
D) Decrease of population
✅
Ans: C
6. If growth rate is 10%, r equals:
A) 10
B) 0.10
C) 1
D) 0.01
✅
Ans: B
7. Logistic curve method assumes:
A) Infinite growth
B) Linear growth
C) Saturation population
D) Sudden fall
✅
Ans: C
8. Population is generally forecasted for:
A) 5 years
B) 10 years
C) 30–50 years
D) 100 years
✅
Ans: C
9. Which gives minimum future population?
A) Geometric
B) Incremental
C) Arithmetic
D) Logistic
✅
Ans: C
10. Growth rate is usually calculated per:
A) Year
B) Month
C) Decade
D) Day
✅
Ans: C
11. If population data is given at 10-year intervals, the
growth rate is calculated per:
A) Year
B) Month
C) Decade
D) Census
✅
Answer: C
📝
Explanation: JE exams always take growth rate per decade
12. Which method is most suitable for an old and fully
developed city?
A) Geometric
B) Incremental
C) Arithmetic
D) Logistic
✅
Answer: C
📝
Explanation: Old cities show nearly constant increase
13. In geometric increase method, if growth rate is 25%,
r equals:
A) 25
B) 0.25
C) 2.5
D) 1.25
✅
Answer: B
📝
Trap: Never use percentage directly
14. Population of a town increased from 20,000 to 24,000
in one decade. Growth rate is:
A) 2%
B) 4%
C) 20%
D) 40%
✅
Answer: C
📝
Explanation:
Increase = 4000 → (4000 / 20000) × 100 = 20%
15. Incremental increase method considers:
A) Only average increase
B) Only growth rate
C) Increase of increase
D) Decrease in growth
✅
Answer: C
16. Which method gives the most realistic population
value?
A) Arithmetic
B) Geometric
C) Incremental
D) Graphical
✅
Answer: C
17. Logistic curve method is based on assumption of:
A) Linear growth
B) Constant increase
C) Saturation population
D) Zero growth
✅
Answer: C
18. If population is forecasted for 30 years using decade
data, value of n is:
A) 1
B) 2
C) 3
D) 30
✅
Answer: C
📝
Trap: 30 years = 3 decades
19. Arithmetic increase method always gives population:
A) Maximum
B) Minimum
C) Average
D) Random
✅
Answer: B
20. Which method assumes compound interest law?
A) Arithmetic
B) Incremental
C) Geometric
D) Logistic
✅
Answer: C
21. If average increase per decade is zero, the town is:
A) Growing
B) Rapidly growing
C) Stable
D) Declining
✅
Answer: C
22. A rapidly industrializing city should use:
A) Arithmetic method
B) Incremental method
C) Geometric method
D) Graphical method
✅
Answer: C
23. Which of the following is NOT a population
forecasting method?
A) Arithmetic
B) Geometric
C) Incremental
D) Rational
✅
Answer: D
24. In arithmetic method, unit of increase (I) is:
A) % per year
B) % per decade
C) Population per decade
D) Population per year
✅
Answer: C
25. A town has population data for 1990, 2000, 2010.
Average increase is calculated based on:
A) Only first decade
B) Only last decade
C) All decades
D) Future decade
✅
Answer: C
26. If growth rate is negative, population is:
A) Increasing
B) Stable
C) Decreasing
D) Saturated
✅
Answer: C
27. Geometric method should NOT be used for:
A) New town
B) Rapid growth city
C) Old developed town
D) Industrial city
✅
Answer: C
28. In incremental increase method, ΔI represents:
A) Population
B) Growth rate
C) Average increase
D) Average incremental increase
✅
Answer: D
29. If past population shows irregular trend, best method
is:
A) Arithmetic
B) Geometric
C) Incremental
D) Graphical
✅
Answer: D
30. Which statement is TRUE?
A) Arithmetic method gives maximum population
B) Geometric method assumes linear growth
C) Incremental method is modification of arithmetic method
D) Logistic method is most commonly used in JE exams
✅
Answer: C
📝
Explanation: Incremental method = Arithmetic + correction factor