What are the degrees of falsifiability?

A potential falsifier of a theory is any potential observation statement that would contradict the theory; for instance 'Peter the swan is black' is a falsifier of the hypothesis that 'all swans are white'. One theory can be more falsifiable than another if the class of potential falsifiers is larger. Therefore scientists prefer simple theories, because they are better testable. On the basis of not falsifiable theories Meehl criticized much psychology. 'Group A will score differently from Group B' also rules out virtually nothing and is a very weak theory. 

A theory can gain in falsifiability not only by being precise but also by being broad in the range of situations to which the theory applies. The greater the universality of a theory the more falsifiable it is, even if the predictions it makes are not very precise. 

Revisions to a theory may make it more falsifiable by specifying fine-grained causal mechanisms. As long as the steps in the proposed causal pathways are testable, specifying the pathway gives you more falsifiers.

Psychologists sometimes theorize and make predictions by constructing computational model, this is a computer simulation of a subject. In order for the model to perform, the free parameters have to be set in particular values, but you can't directly observe the values of these parameters. 

With computational models it can be difficult to predict how the model will do just by thinking about it. The model has to be run and its behaviour observed and analyzed. Often modellers just try to find any set of parameters values that fits the data. If the best-fitting model of each fitted about as wel, the modeler may conclude that there is no reason to prefer one model than another. Popper's idea showed that is inadequacy in simply finding the best-fit models. If a model has passed more severe tests it was more falsifiable to begin with. 

A theory that allows everything explains nothing, the more a theory forbids, the more it says about the world and the 'empirical content' of a theory increases with its degree of falsifiability. But also, the more falsifiable a theory is, the more it is open to criticism. So, the more falsifiable, the faster you can make progress, given progress comes from criticism. 

Popper stated that good science shows itself not just by the simple literal form of its theories, but also by the nature of history of its theories leading to the current proposals. When you have the hypothesis 'all swans are white', you can find one exception 'Peter the swan is black'. This amendment to the theory is adhoc and decreases the falsifiability and is unsatisfying. Popper proposed that revisions and amendments should always increase falsifiability. In psychology, attempts to save theories and to not make new measures are often called 'post hoc' .

Falsifiability: too strong a criterion or too weak?

There are two criticisms of Popper's approach:

1. No theory is falsifiable at all 
2. All theories are falsified anyway

Critics often focus on the fact that accepting an observation statement involves accepting various levels of theory as well as the theory under test. There is no general method of determining which of the theories should be rejected when an apparent falsification occurs. When you have a falsification, how do we know which component of the system to reject? This is a widely recognized problem and is called the Duhem-Quine problem.