How about to check the number of ITA sent out during that time compared to the number of ITA for the same period of time in 2016. (as they have issued more ITA in first 9 months of 2017) significantly more than in whole year 2016. That alone will indicate the increasing amount of interest there.
In addition your estimation is a decent increase already from 50k to 61k is a 20% yearly increase which is not little at all.
In addition ITA is influenced:
1. The size of the yearly quota
2. The size of the average family (as the quota is speaking about nr. of immigrants but not about nr. of applications)
3. The % of not used. cancelled or rejected application. (simply said, the more prepared people are and the less mistakes they made in their application, the lower this % will).
While you know the quota in advance, the size of the families and the % of failure is only known at the end of each measured period (for example every quarter or every year). So when they are issuing ITA, they are using the % from the past in order to project the correct amount for the future. Hence if they see, that at the end of next measured period they have invited too few people they will increase the invitation (what we have seen in the first half of 2017 when they compared with 2016). But if they see that they have invited too many, they will cut down.
Eventually the % of failures tends to go down (as people will get familiar with the new system which will result in less mistakes in the applications).
So the positive effect would be the quota increase while the negative effect would be any stagnation or decrease of this quota. Same goes for the family size.
