With my pleasure, and all the best.

Martin &and Rubin (1995) did a series of studies to test the validity of the Cognitive Flexibility Scale (CFS).  You might research their article:

Martin, M. M., & Rubin, R. B. (1995). A new measure of cognitive flexibility.  Psychological Reports, 76, 623-626.

This might be a good starting point.

Dear Cha Weng:

You ask for a cognitive flexibility scale. The references Vladimir and Will sent to you are worth of noting and will be of much help.

As far as I know the concept of cognitive flexibility lies at the heart of well-known theories of cognitive development (e.g., Piaget’s theory) moral development (e.g., Kohlberg’s theory), interpersonal development (e.g., Selman’s theory), just to mention three examples. 

I hope that my considerations below may be also of some help to you. .

Let me start by saying that I am not much in favor of standardized scales because, among other things, in those scales, generally Likert scales, one can get a given score (IQ = 100, for example) by answering differently to the several items the respective scale contains (e.g., WAIS or WISC). Because of this, the same score (e.g., 100) can have different psychological meanings. In this vein, let me say that Piagetian tasks -- not mental tests or scales -- can also be used for assessing one’s cognitive flexibility. Note that in Piaget’s theory, intelligence is defined as one’s ability to actively adapt to the changing physical and social environment. Thus, the more one’s thinking is flexible, the more intelligent one is. To substantiate my argument, I provide you with a task which appears in his book: Possibility and necessity. Vol.1: The role of possibility in cognitive development. Minneapolis: University of Minnesota Press (original work published in 1981)  

Here it goes a succinct description of the task as well as the way one may respond on that task and the cognitive classification of individuals’ on the task.  The interviewer or experimenter (E, say) starts by showing the interviewee (I, say) -- be s/he a child, adolescent, or even adult -- a little part of, for example, a red pencil. The remaining part of the pencil is hidden behind a book, for instance. After this, E asks the interviewee the following: “Which is it the color of the hidden part of the pencil? Why is this so?” Children under 5-6 years of age, generally say that the hidden part is red because the visible part of the pencil is also red.  E: “Could it – the hidden part-- be of a different color, yellow, for example?” I:: “No. It has to be red”. This is a typical preoperational answer. The child at hand shows no cognitive flexibility in that s/he is not capable of imaging a possibility different from that s/he is really seeing (i.e., the visible part is red). In Piagetian terms, this child confounds the real (what s/he sees) with the possible (what s/he has to imagine) and the necessary, pseudo-necessary in this case (“It has to be read”-- what has not to be necessarily the case, for the hidden part of the pencil has not to be necessarily red).

Between 5-6 and 8-9 years of age, when confronted with such possibility (i.e., “Could it be of a different color?”) -- which is different from reality (e.g., the visible part of the pencil is red) -- children’s generally admit that the invisible part of the pencil could be yellow, blue, white, black, but not much more colors than theses. When asked if the color of the invisible part could differ from its visible counterpart, much more than 10 or 20 colors, for instance, the child often says: “No. Twenty colors are already many colors”. In Piagetian terms, this would be a concrete operational answer, an answer that shows more cognitive flexibility than that present in a preoperational response. Even so, this cognitive flexibility is limited or constrained (e.g., “No. Twenty colors are already many colors. I also know only of 8 colors”)   

When confronted with the possibility mentioned above (“Could it be of a different color rather than red?”), adolescents and adults, generally say, “Of course, it can be of a different color from red”. E: “How many different colors?” I: “As many colors as those I know, and even don’t know”. This would be a formal operational answer, an answer that reveals a huge cognitive flexibility in that it shows that, say, the world of possibilities is infinite and goes far well beyond the world of realities.

As this example clearly shows, almost any Piagetian task (e.g., conservation tasks, the pendulum task, the balance task, and so forth) can be successfully used for assessing one’s cognitive flexibility, with the advantage that these tasks are grounded on a strong theory of intelligence and cognitive development, this not being the case of many Likert scales.   

Of course, one can employ both Likert scales and Piagetian tasks in order to have a broad picture of one’s cognitive flexibility.

I wonder if you are acquainted with Herman Witkin’s cognitive styles, field dependency and field independency, and his Rod and Frame Test. If this is the case, you know that the more individuals are field independent the more flexible in cognitive terms they are. This would other possibility to assess individuals’ cognitive flexibility.             

Best regards.