Estimates Forecast Projections

7/25/2019 Estimates Forecast Projections

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Estimate

An indirect measure of a past or presentcondition that can be directly measured.

For example, you can estimate current ridershiplevels on a bus route by counting riders on asample of days during a given month.

"estimates" are not made for future conditions.Also, unlike projections and forecasts,estimates are directly measured and thereforeare based upon what is known and

can bechecked for accuracy.



Projection

A conditional "if, then" statement about the future.

For example, if a city grew by ! between #$! and##! from !,!!! to ,!!!, then given a continuedten year growth rate of !, that city will have a

population of %,!! in the year %!!!.

A projection is the most popular and most defensible"crystal ball" method because it is impossible tomake an incorrect projection if one&s calculations arecorrect.

A projection simply states that "if the followinghappens, then the future will be this way".

'f the following does not happen, the analyst has not



Forecast

A judgmental statement that is the "best guess"

about a future condition( a forecast is the analyst&sdetermination of the most likely future.

For example, given a range of projections, an analystmay decide that because of the strong economy and

growing popularity of the )*+ region , elhi-spopulation is expected to grow by ! between %!!and %!!. /his would be that analyst&s best guessabout the future, not a conditional statement aboutfuture conditions.

0owever, only very rarely are forecasts veryaccurate, and more often than not unforeseen 1andoftentimes unforeseeable2 events result in wildly

inaccurate forecasts



Population Projection Methods

• 5raphical

• ecreasing rate of growth

•

6athematical or 7ogistical• +atio and *orrelation

• *omponent

•8mployment forecasts



Factors a9ecting changes in population are:

• increase due to births

• decrease due to deaths

• increase; decrease due to migration

•

increase due to annexation• /he present and past population record for the city

can be obtained from the census population records.

• After collecting these population <gures, the

population at the end of design period is predictedusing various methods as suitable for that cityconsidering the growth pattern followed by the city.



AR!M#!$A% &$R#A'# M#!()

• /his method is suitable for large and old city withconsiderable development.

• 'f it is used for small, average or comparatively new cities,it will give low result than actual value.

• 'n this method the average increase in population perdecade is calculated from the past census reports.

•

/his increase is added to the present population to <nd outthe population of the next decade.

• /hus, it is assumed that the population is increasing atconstant rate.

• 0ence, d=;dt > * i.e. rate of change of population with

respect to time is constant.• /herefore, =opulation after nth decade will be Pn* P + n$

• 3here, =n is the population after n decade and = is presentpopulation.





-#(M#!R$A% &$R#A'# M#!() 1?+ 58?68/+'*A7=+?5+8@@'?) 68/0?2

• 'n this method the percentage increase in populationfrom decade to decade is assumed to remain constant.

• 5eometric mean increase is used to <nd out the futureincrement in population.

• @ince this method gives higher values and henceshould be applied for a new industrial town at thebeginning of development for only few decades.

• /he population at the end of nth decade =n- can be

estimated as: Pn * P /+ -0/112 n

3here, 5 > geometric mean 12

= > =resent population

n > no. of decades.





&$R#M#&!A% &$R#A'# M#!()

• /his method is modi<cation of arithmetical increase method

and it is suitable for an average si3e town under normalcondition where the growth rate is found to be in increasingorder.

• 3hile adopting this method the increase in increment isconsidered for calculating future population.

• /he incremental increase is determined for each decadefrom the past population and the average value is added tothe present population along with the average rate ofincrease.

0ence, population after nth decade is Pn * P+ n4 + 5nn+/20678

3here, =n > =opulation after nth decade

B > Average increase

C > 'ncremental increase





-RAP$A% M#!()

• 'n this method, the populations of last few decades arecorrectly plotted to a suitable scale on graph.

• /he population curve is smoothly extended for gettingfuture population.

• /his extension should be done carefully and it reDuiresproper experience and judgment.

• /he best way ofapplying this method isto extend the curve by

comparing withpopulation curve ofsome other similarcities having the similargrowth condition.



MA'!#R P%A& M#!() 9or new cities2

• /he big and metropolitan cities are generally notdeveloped in haphaEard manner, but are planned andregulated by local bodies according to master plan.

• /he master plan is prepared for next % to ! years forthe city.

• According to the master plan the city is divided into

various Eones such as residence, commerce andindustry.

• /he population densities are <xed for various Eones inthe master plan.

•

From this population density total water demand andwastewater generation for that Eone can be worked out.

• @o by this method it is very easy to access precisely thedesign population.

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Estimates Forecast Projections