In other scenarios, there are factors whose levels are hard or expensive to change.
This selection may be based on considerations of cost or availability some types of data are hard or expensive to get; cheap and accessible methods may be better suited to testing the causal influence of some variables than others.
"It can be hard and expensive to get noticed online now.
However, such efforts would be hard, intrusive, and expensive.
As Congress and the Bush administration return to the domestic agenda, almost no issue will be harder, more expensive or more divisive to deal with than health care.
But a better bug might be engineered.Putting a viral genome together from scratch would not be that hard or expensive (perhaps $1m), and it could probably be done without anyone noticing.
Recruiting experienced players would surely be harder and more expensive.
However, if training data was particularly hard or expensive to acquire in a new country or if slightly lower accuracy is enough for the application at hand, 4 out of 5 of the algorithms we tried reach good level of accuracy with smaller training set.
But much of the remaining British oil is hard and expensive to extract.
Having babies is hard, often expensive, and it doesn't always work.
