Background: I have been given a yearly forecast for a product. I have also looked up actual sales for this product for the first half of the year. The actual demand seems "spikier" than the forecast.

The forecast accounts for promotions and seasonality.

I do not know how the forecasts are developed but they seem to be more accurate when the demand spikes. I also suspect that the forecast becomes less accurate as time goes on because it is created for the entire year and not updated until next year.

Challenge: How can I study the behavior of the forecast compared to the actual numbers and then adjust for it?

Raw data:
Code: 
Forecast               Actual
19183.3272	16653
32208.876	28931
27491.9968	25029
25056.3944	22153
28525.9616	26054
28492.3544	28823
24723.8336	24720
23773.9704	22583
29041.9408	26421
31325.5584	28772
34693.1336	34505
26171.6184	21967
22321.3368	17327
22263.4856	18404
27535.8032	27976
28900.9912	26802
26843.5952	21160
27597.8344	22652
26449.8392	21927
28135.048	24426
28934.4312	24383
25587.4216	24734
24521.856	20429
25059.7384	22658
22401.9272	18146
25519.5384	20982
25675.7032	20326
27549.3464	24476
25417.3792	22086
29116.0104	25090
29281.8728	22834
27603.8536	20418
30142.4512	25023
29374.1672	
29615.1024	
24717.48	
31143.1432	
32460.512	
28484.3288	
24079.7792	
29370.9904	
30232.2376	
27689.46	
24524.0296	
26324.1048	
25655.6392	
21960.5192	
21334.188	
26529.4264	
24357.8328	
24102.184	
19934.2224
Same numbers in list
Code: 
Forecast
19183.3272
32208.876
27491.9968
25056.3944
28525.9616
28492.3544
24723.8336
23773.9704
29041.9408
31325.5584
34693.1336
26171.6184
22321.3368
22263.4856
27535.8032
28900.9912
26843.5952
27597.8344
26449.8392
28135.048
28934.4312
25587.4216
24521.856
25059.7384
22401.9272
25519.5384
25675.7032
27549.3464
25417.3792
29116.0104
29281.8728
27603.8536
30142.4512
29374.1672
29615.1024
24717.48
31143.1432
32460.512
28484.3288
24079.7792
29370.9904
30232.2376
27689.46
24524.0296
26324.1048
25655.6392
21960.5192
21334.188
26529.4264
24357.8328
24102.184
19934.2224


Actual
16653
28931
25029
22153
26054
28823
24720
22583
26421
28772
34505
21967
17327
18404
27976
26802
21160
22652
21927
24426
24383
24734
20429
22658
18146
20982
20326
24476
22086
25090
22834
20418
25023