Difference in literacy abilities in Primary School children whether they are born at the start of an academic year compared with the end

My dissertation focuses on the academic abilities of primary school children and how or if age and birthdate affect a child's competence in literacy (where age and birthdate relate to the notion of whether a child has a birthday in September or October as opposed to July/August). My hypothesis is that children born at the end of the year struggle academically, and this has been noted in the literature. ‘Professionals working in the field of education have long been aware of the disadvantageous position that summer-born children (the youngest in the school year) may be placed in within the education system.’ (Gledhill:2008:1)
In order to reach a conclusion I have decided to assess real samples of writing to prove or disprove the hypothesis. I have two sets of sample data from two year groups: Year 3 and Year 4 with 10 samples from each year. We can see this in the quote. If the hypothesis is proven, it is clear that this would be disadvantaging children with nothing really being done to change the way the school year works. Provisional findings do suggest that there is evidence of a difference in performance between the beginning of the year and end of year, especially within their writing and spelling skills