1. A method for determining motion vectors between a current frame and its preceding frame of video signals, wherein the current frame is divided into a plurality of search blocks of an identical size and the preceding frame is divided into a corresponding number of search regions, each search region being further divided into a multiplicity of candidate blocks of said identical size, which comprises the steps of: (a) generating motion vectors, each of which represents the displacement of pixels between the search block and each of the multiplicity of candidate blocks in the search region corresponding to said search block; (b) producing prediction signals, in response to the motion vectors generated in step (a), each of said prediction signals corresponding to the pixel data for each of the candidate blocks; (c) subtracting each of said prediction signals from the pixel data of the search block, thereby providing the error signals; (d) calculating an error function and an index of correlation for each of said error signals, wherein i) said error function is a mean absolute error defined as: ##EQU9## wherein E.sub.i is the mean absolute error of an error signal representing a block i; I(s,t) represents the pixel data at column s and row t in the block i of M.times.N pixels; and ii) said index of correlation is an average correlation coefficient defined as: ##EQU10## wherein C.sub.i is the average correlation coefficient of the error signal representing the block i; C.sub.i,h, a horizontal correlation coefficient of the block i; and C.sub.i,v, a vertical correlation coefficient of the block i, C.sub.i,h being defined as: ##EQU11## wherein N is the number of rows in the block i; M is the number of columns in the block i; m is a positive integer ranging from 1 to (M-1); I(s+m,t) is the pixel data at column (s+m) and row t; and mean.sub.t is an average pixel data in row t defined by ##EQU12## and C.sub.i,v being defined as: ##EQU13## wherein m' is a positive integer ranging from 1 to (N-1); I(s, t+m') represents the pixel data at row (t+m') in column s; and mean.sub.s is an average pixel data in column s defined by ##EQU14## (e) selecting one or more error signals capable of providing a least number of bits after compression such that said one or more error signals comprise an error signal having a minimum error function and are selected in the ascending order of the error functions, other selected error signals than said error signal having the minimum error function have indices of correlation larger than that of said error signal having the minimum error function, and the total number of said one or more error signals does not exceed a predetermined number, to thereby provide one or more motion vectors, each of which represents each of said selected one or more error signals; (f) producing prediction signals, each of the prediction signals corresponding to the pixel data of a candidate block represented by each of the motion vectors produced in step (e); (g) subtracting each of the prediction signals produced in step (f) from the pixel data of the search block, thereby reconstructing said one or more error signals selected in step (e); (h) transform-coding each of said reconstructed one or more error signals by using the discrete cosine transform, thereby producing a set of transform coefficients; (i) quantizing said set of transform coefficients into a set of quantized transform coefficients; (j) scanning said set of quantized transform coefficients, to thereby produce a sequence of quantized transform coefficients; (k) statistically coding said sequence of quantized transform coefficients by using the variable length coding, to thereby provide one or more statistically coded error signals; (l) counting the number of bits for each of the one or more statistically coded error signals; (m) comparing the numbers of bits counted in step (l) with each other; and (n) selecting an error signal containing the least number of bits, to thereby provide a motion vector corresponding to said selected error signal as the motion vector for the search block.