# Choosing between Holt-Winters additive and multiplicative methods

I am attaching the question, I am solving for context.

The sales of the average price of Fiat cars sold in a garage in the Belgian province of Limburg for each month are listed below. The foreman, Mr Verstraten has this data in the period 2010-12.

Mr Verstraten intends to forecast the average car sale prices in each month. Evaluating the data found by Mr Verstraten in 2010-11, find the preliminary estimates for seasonal factors, trend and base. The sales of the average price of Fiat cars sold in a garage in the Belgian province of Limburg for each month are listed below. The foreman, Mr Verstraten has this data in the period 2010-12.

Mr Verstraten intends to forecast the average car sale prices in each month. Evaluating the data found by Mr Verstraten in 2010-11, find the preliminary estimates for seasonal factors, trend and base.

I understand that there is Holt-Winters additive method and Holt-Winters multiplicative method. Now since I am working with monthly data, I know that we are inclined to opt for the additive method "when the seasonal variations are roughly constant through the series ". We would opt for the multiplicative method: when the seasonal variations are changing proportional to the level of the series. I would consider that the question would require me to use the additive method owing to the data that I have collected. And additionally typically in examples of the multiplicative method that I have done in the past there are volatile changes between periods.

For reference I am using this website which elucidates the notion of which method to elect: https://www.otexts.org/fpp/7/5

Here is a time-series plot of the sales for 2010 and 2011:

Does this look like additive seasonality?

• Is there something you don't understand in the reference? Plotting the time series might help you to decide. Apr 21, 2018 at 11:17
• You're welcome. You can post the plot here if you're unsure still. Apr 21, 2018 at 11:37
• imgur.com/a/bwBTkIv Apr 21, 2018 at 12:10
• I've added it to the question. But what exactly are you asking? Apr 21, 2018 at 12:15
• You're welcome. I've made the comment an answer. And edited the question to make it a question. Please remember next time to to make it clear what exactly you need help with. Apr 21, 2018 at 12:37