# Input variable that influences only trend

I need to predict cost for the next 4 weeks. Along with categorical variables (available for future as well), I have a numerical variable (a value from the day before prediction - I don't have future values for this variable), let's call it foo, that is telling me if I am spending too much or too little. Basically, it tells me if I need to increase or decrease the cost in the following days. I am not sure how to approach this. How to incorporate this variable in the model.

I thought that I can I maybe declare a categorical variable for future days that would have value 'grow' for the time that it is relevant (for example first 2 weeks), and for the rest value 'na' (last 2 weeks), since I don't know the trend of cost for them? But then how would I create this variable for the past...

Maybe I just need a multiplier, to raise my cost, or lower them depending on foo value from the day before.

Do you have any suggestions?
PS: I am planning to use LSTM, but I am open to other suggestions.

• Why are you not just simply trying to predict the cost in the next month using regression? Nov 19 '20 at 12:28
• @Taylrl I will start with regression, but I also want to do multivariate LSTM and see if it will outperform the regression. Do you have any suggestions on how to use this foo variable? Nov 19 '20 at 12:53
• I would just include it as another categorical variable Nov 19 '20 at 12:55
• What is foo and why it is not available for the future? I hope it is not calculated based on cost. I mean a source of data leakage Nov 19 '20 at 16:36