AbstractThis thesis is a quantitative research focused on the measurement of the Reliability of the Spanish Version of the Utrecht Early Mathematical Competence Test (Scale A). The Utrecht Early Mathematical Competence Test was constructed in 1994 by Hans van Luit; Bernadette van de Rijt & Albèr H. Pennings, in Netherlands. Initially it had a pool of 120 items. Currently, it is composed by two scales (A and B) each consisting of 40 items.The purpose of this test it to assess the developmental level of early mathematical competence in children ages 4 to 7 years, by evaluating eight mathematical concepts: Comparison of quantitative and qualitative characteristics of objects; classification of objects in class or subclass; correspondence one to one relation; seriation of objects in class or subclass based on criteria; counting words forward and backward; structured counting; resultative counting and general knowledge of numbers.As a primary teacher graduated from the Pontifical University of Ecuador, I have always been concerned about mathematical learning disabilities and I believe that the use of an appropriate assessment tool that helps us teachers to know the mathematical development of kindergarten students can be crucial in preventing future learning problems or inferior performance later. It can also contribute in a significant way to special educational planning and decision-making, for example, in terms of more accurate evaluation and diagnosing, in establishing supportive social networks, and in the designing of appropriate learning materials. Among some instruments used in Ecuador to assess the mathematical skills in children we can find the Spanish version of the Utrecht Early Mathematical Competence Test (Scale A). This version was developed in the University of Cadiz, Spain. Considering that there have been no studies about the reliability of the results obtained in the application of this test in Ecuador, the aim of my project will be to measure the reliability of this version when it is used in Ecuadorian kindergartens. The research is focused on reliability because it reflects the ability of an instrument to perform and maintain its functions in routine. A low punctuation of the reliability of a test can be a sign that the items are ambiguous or that the standard conditions during the administration of the test are failing. In other words, measuring the reliability of the test will show us the consistency of this measure in the Ecuadorian environment.Between the several general classes of reliability estimates, two types of reliability were estimated for this research: The test-retest and the internal consistency. The study was carried out on a sample of one hundred Ecuadorian children, between four and five years old. The children were students of 3 different kindergartens in Quito, which are well known for their good academic level and they are ranked on a medium-high economical level. After applying the statistical techniques to the data collected through the SPSS program, the statistical estimations showed a high degree for both, test-retest reliability and internal consistency reliability. These results of the empirical studies confirmed that the Spanish version of the Utrecht Early mathematical competence is a reliable assessment method for examining the numerical abilities of Ecuadorian preschool children.