Is research experience in a different field relevant for PhD application?

While field-specific research experience is always the best kind, I think there is a role for good research experience in any related field. Close-by disciplines that share similar research methods and approaches will carry much more weight than something much further removed. (For instance, as a chemical engineer, I wouldn't put much stock in somebody doing sociology or clinical medical research, but somebody who had a background in research in applied mathematics or mechanical engineering would be potentially of interest.)

So, if you're going to go into an area of economics where your mathematical statistics background will be of benefit, I think that will (or at least should) be viewed favorably by an admissions committee.

That said, you will want to make sure that your research supervisor understands that you're applying to economics programs, and tailors her letter of recommendation accordingly. Also, if you have a lot of experience in a different field, you will need to make clear to the admissions committee why it is you want to pursue the new field.

A graduate program in any research field considers a variety of criteria for each applicant. The contents of a good application to graduate school have been addressed here before.

Most admissions committees look at several criteria, which are often weighted, but none weighted so high as to dominate the whole decision. A strong showing in one category can offset a weaker showing in another. Among them are:

• Basic knowledge of the field: You demonstrate this by completing a Bachelor's Degree in the field or a related field, or presenting evidence that you will do so by the time you matriculate. For example, in your question you mention that you are majoring in economics and want to go to graduate school in economics. No conflicts there. If you were majoring in philosophy and wanted to pursue a Ph.D. in chemistry, you would be a harder sell. What constitutes a related field may vary, but I know that a B.S. in mathematics can get you into graduate programs in engineering.
• Academic ability: You demonstrate this through your grades in your courses, especially those relevant to the graduate program you want to enroll in, and through your scores on whatever standardized tests the program asks you to take before you apply.
• Research potential: You demonstrate this one of two ways. If you were lucky enough to publish your research as an undergraduate, then the committee can assess what you did directly. If not, then you need one or more letters of recommendation from someone who has supervised you in a research setting. The admissions committee wants evidence that you have certain qualities that make good researchers, regardless of discipline: curiosity, critical-thinking, creativity, strong work ethic, passion or drive, etc. A good research experience in another field is worth much more than a bad one in your own field.
• Communications skills: You demonstrate these through your personal statement (or essay or letter) that you must write. Spelling and grammar errors or worse writing problems in this document are more damning than a bad letter of recommendation!
• And as JeffE mentions in his answer to a related question: No Red Flags