Forecasting future trend

I am working with data, not financial or sales data..the number are persons who come for shopping each month...i need to predict what will be the future growth (positive or negative) in terms of month, quarter & year. i know by looking at figure that growth rate is negative.
there are seasonal factor which affects the turnout...and should be considered while predicting the furture.
seasonal month:
Sep & Mar 2009
Aug & Mar 2010
Aug & Feb 2011
Aug & Feb 2012

Month No. of visitor
Feb-09 129
Mar-09 147
Apr-09 151
May-09 150
Jun-09 118
Jul-09 86
Aug-09 192
Sep-09 213
Oct-09 100
Nov-09 115
Dec-09 122
Jan-10 100
Feb-10 113
Mar-10 104
Apr-10 109
May-10 106
Jun-10 107
Jul-10 124
Aug-10 107
Sep-10 58
Oct-10 64
Nov-10 75
Dec-10 80
Jan-11 51
Feb-11 101
Mar-11 85
Apr-11 68
May-11 58
Jun-11 50
Jul-11 79
Aug-11 122
Sep-11 40
Oct-11 51
Nov-11 65
Dec-11 77
Jan-12
Feb-12
Mar-12
Apr-12
May-12
Jun-12
Jul-12
Aug-12
Sep-12
Oct-12
Nov-12
Dec-12

i searching for the solution for some days,,,,, but get confused wht forumula or function i use..... !!

Originally Posted by zbor

Upload example workbook and write expected results and how you got them.

For example: Q2 of 2012 is based on average of Q2 2009, Q2 2010, Q2 2011

or

Q2 of 2012 is based on Q2 2011 and Q1 of 2012 etc.

Something like this:

Actually the reason for predicting future is to known how many visitors we can expect in coming months, quarter, and year; on basis of past trend.
You brilliantly put data in quarter and half yearly frequency, but not filled monthly projection... i assumed u simply averaged the values. i read some where not to use simple average function when data fluctuates frequently they advises to use geometric average (geomean) or logest or Growth function in excel....

whatever ever function is being used but most important is to predict future values more closer to actual....also consider that seasonal factor has affect on visitors..& seasonal month cant be considered as normal month.

i hope this explanation and attached file wil make u understand .....

Originally Posted by Richard Buttrey

I'm not sure we can help very much unless we know more about your business and what variables are in place. Like for instance is the weather/temperature likely to affect the number of visitors, would discretional spend ability be a factor and maybe several others.

Looking at a rolling 3 month average the trend has been downward since 2009 as you have identified, but ticked up in the last three rolling averages. Do you have any information which explains this - visitor surveys for instance.

Unless we know a lot more it's impossible to give you an answer.

Incidentally would you change your location to something more meaningful. This often helps us when responding - particularly where any date arithmetic is involved since your locale gives us clues as to your formatting standard.

to be exact the variable that effects customer turnout are annual festival just like u have Christmas or Easter where impact on sales are totally different from normal months... so every year we have 2 months of festival season based on lunar calendar..
i m interested to evaluate whether customers are going to increase or decrease in future & chances of growth...
Also look tht i only put real (exact customer) data, monthly basis from Feb-09 to Feb-12, quarterly and half yearly averages are done by Zbor from same forum. I m looking to know wht would be the customer projection for march onwards or to be more exact in which month we have to decide to close the business by looking at customer numbers.