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## 2.1 Future Assumption

Future assumption is the assumption to be made about the source St+1 of the target instance Xt+1.

We identify three types of choices here.

1. Assuming that St+1 = St.

2. Estimating the source based on Xt+1.

3. Predicting the change.
The first option, assuming St+1 = St, is the most common among concept drift, although rarely explicitly stated. It is assumed that in the nearest future we will see the data coming from the same source as we saw in the near past.

The second option utilizes information from the unlabeled target instance Xt+1. Estimation of the source is usually done by measuring the distance between Xt+1 and historical reference instances.

The algorithms presented in [148, 123, 37, 97] use this future assumption.

Generally in concept drift problem the future data source is not known with certainty. However, there are methods using trainable prediction rules, to estimate the future state and incorporate that estimation into the incremental learning process. The algorithms using future predictions are presented in [24, 157, 161, 21].