Concept: Birth / Grade Cohorts

Introduction

Children can be categorized into groups, or cohorts, often according to their year of birth or grade in school. This is useful when studying the developmental progress of children, as they can be compared to others in their particular cohort. This comparison is especially common in education research, when looking at how children progress through their schooling at different rates. In Manitoba research, birth dates have been particularly important for looking at date of school entry and grade retention.

Birth Cohort - Definition and Measurement

Birth cohort (also known as age cohort ) is defined by birth in a particular year, or a range of birth years.

At MCHP, birth cohorts are initially constructed from the mother-child hospital record, which records each hospital-birth in Manitoba. Following each birth, Manitoba Health assigns each newborn a unique Personal Health Identification Number (PHIN) . MCHP researchers can also search the registry for infants who are born outside of a hospital and, therefore, are not included in the mother-child hospital record.

Grade Cohort - Definition

Children can be categorized into groups, or cohorts, according to their grade level in school. Grade cohort is defined by being in a particular grade in a particular year, or in a particular range of grades. Grade cohort can be distinguished from birth cohort, although an overlap between the two cohort types is likely (see Age-Appropriate Grade Placement below). In longitudinal studies, grade cohorts may be followed over time. Studies may compare children to others in their cohort, or make comparisons between those in different cohorts.

For example, MCHP researchers followed a grade cohort that was enrolled in Grade 9 in the 1997 academic year (1997/1998 school year). The grade cohort was followed to determine whether students had graduated, remained in school, or withdrawn (Guevremont, Roos, & Brownell) .

Age-Appropriate Grade Placement - Definition

In Manitoba, age-appropriate grade placement depends on both the month and year in which a child is born. Currently, for a child to be age-appropriately placed in Grade 1, the child must turn 6 by December 31st of their Grade 1 year. The age-appropriate range for children entering Grade 1 would be from the youngest age-appropriately placed children who turned 6 on December 31st of their Grade 1 year to the oldest age-appropriately placed children who turned 6 on January 1st prior to entering Grade 1. In other words, the appropriate age for a child to enter Grade 1 would be in the Fall of the calendar year, as opposed to the school year, in which the child is expected to turn 6.

With age-appropriate grade placement described, the following table identifies the corresponding year of birth (birth cohort) and the expected school year that this cohort will be in grade 12:

Birth Cohort - Year of Birth	Expected School Year in Grade 12
1978	1995/1996
1979	1996/1997
1980	1997/1998
1981	1998/1999
1982	1999/2000
1983	2000/2001
1984	2001/2002
1985	2002/2003
1986	2003/2004
1987	2004/2005
1988	2005/2006
1989	2006/2007
1990	2007/2008

Grade Cohort and Birth Cohort

The majority of students will be in a given grade at the age-appropriate time; hence, grade cohort and age-cohort largely overlap. MCHP researchers have encountered this overlap when examining student populations, for a given academic/school year, using both age cohort and grade (Brownell, Roos, Fransoo, Roos, Guevremont, MacWilliam, Yallop, & Levin, 2006) .

For example, 12,574 students in Manitoba were in Grade 3 and wrote the Grade 3 standards tests in the 1998 academic year (1998/1999 school year). Some of these students may not have been born in Manitoba, and some of these students may not be in Grade 3 at the age-appropriate time. Children born in 1990 (in the 1990 birth cohort) would be in Grade 3 and taking the test at the age-appropriate time. The 1990 birth cohort included those born in Manitoba in 1990 and still residing in Manitoba during the 1998 academic year (1998/1999 school year). There were a total of 12,574 students in Grade 3, whereas there were 13,282 children in the 1990 birth cohort. Although the overlap between these two cohort types was large, (10,200 students overlapped), the two populations were not identical.

Different, but overlapping populations were also observed for the Grade 12 standards tests for the 2001 academic year (2001/2002 school year). A total of 11,750 students in Grade 12 wrote the standards tests, but a total of 12,874 youth were in the 1984 birth cohort (who would be at the age-appropriate time to be in Grade 12). The overlap between the two cohort types was less for this higher grade (7,500 students overlapped).

History of Cut-off Dates for School Entry

From 1979 to 1996, the Public Schools Act of Manitoba stated that a child had the right to enroll in Grade 1 in the Fall session (beginning in September) if he or she was 6 years of age or would turn 6-years-old within 12 weeks after September 1st (i.e., December 1st was the cut-off date for the child to reach the minimum age for entry into Grade 1). School divisions were, however, allowed to use their discretion to extend this date to December 31st, if they deemed it to be appropriate. It is unknown how many school divisions implemented such extensions.

In 1996, the Public Schools Act of Manitoba changed the official cut-off date to December 31st for all school divisions. Therefore, as of 1996, children in Manitoba must reach age 6 by December 31st of their Grade 1 year in order to be able to enroll in that Fall session.

November/ December Births

The age-appropriate time for a child to begin Grade 12 would be September of the calendar year, as opposed to the school year, in which the child has turned or is expected to turn 17. For example, the age appropriate time for children born in 1980 to enter Grade 12 would be September, 1997.

Children born in November and December are more likely than those who are born earlier in the year to be in a lower grade than what is deemed to be the "age-appropriate" one. For example, in 1985, 33.09% of children born in December and 25.57% of children born in November entered Grade 12 later than would be considered age-appropriate; whereas, only 11.31% of children born in January entered Grade 12 later than would be expected.

Some of this discrepancy may be due to cut-off dates for school entry (see "History of Cut-Off Dates for School Entry" above). Some children born in November and December started school with their birth cohort and some started the following year. MCHP researchers do not have the enrollment data for the 1989/1990 school year, from which could be determined how many children born in 1984 were delayed in starting Kindergarten or Grade 1.

Using available enrollment data, only approximately 3% of children in Manitoba, and 2% of children in Winnipeg, who began Kindergarten between 1998-2003, began school later than their (same birth-year) cohort. These percentages, however, remained similar (1-2% differences in frequencies) when children born in December were not included in the analyses.

Notes

1. The distribution of pass/fail results for Grade 12 students who wrote the Language Arts (LA) standards test (see the Education Indices concept for more information) was similar across academic years. These results remained similar when children born in December were eliminated from the analyses.
   
   
2. The Oreopoulos et al. (2008) data sample includes more than 96 percent of all children born in Manitoba in 1978-82 and 1984-85 and more than 99 percent of this group remaining in the province up to June of their 18th year. The cohort born in 1983 was not included because grade 12 provincial tests were not given in the school year 2000/2001 (when the 1983 birth cohort would be expected to be in grade 12).