A story in the New York Times dated March 26, 2007, entitled, Poor Behavior Is Linked to Time in Daycare sounded pretty scary, so I looked into it in more detail. This is an issue that worries a lot of parents. They apparently turn to the Internet for information; my previous post on the topic is the fifth ranked entry page on my blog.
The New York Times story was pretty weak, as there were some details either missing or garbled, so I found the article on line. It was recently published in the March/April 2007, issue of Child Development. If you don't subscribe to the journal, it's $29.00 to get access. Seems kinda steep to me. Fortunately, ScienceDaily has a good summary of the article on line.
The study is part of a larger multi-site research project led by the National Institute of Child Health and Human Development (NICHD). The lead author was Jay Belsky, a somewhat controversial researcher who has been raising questions about the effects of childcare for years. I think he's gotten a bad rap. As we put more and more children into child care, we are conducting an experiment on our children and Belsky has been looking at the outcome of that experiment. He's asking questions that need to be answered. The problem is that his results are often complicated and get oversimplified in the press, leading to unnecessary huffing and puffing. These data are a classic example.
The study analyzed data on 1,364 children, who NICHD had tracked from birth. This is not a representative sample. They had recruited the families through hospital visits shortly after birth and collected data on academic achievement, intellectual, and social development from birth to the present day. This is a longitudinal study, which means the children were repeatedly evaluated on the same instruments, so they have a picture of how the children have changed over several years.
During the study, researchers measured the quality, quantity and type of child care the children received from birth until they were 54 months old. Child care was defined as care by anyone other than the child's mother that was regularly scheduled for at least 10 hours per week. This included care by fathers, grandparents and other relatives.
The researchers then evaluated the children's academic achievement, cognitive (intellectual) functioning from kindergarten through fifth grade and social development through sixth grade. Other factors, such as parenting quality and the quality of classroom instruction, were also measured. These other factors were taken into account when examining the association between early child care and children's subsequent development. The study tracked children's experience in child care. It was not designed to determine cause and effect and so could not demonstrate conclusively whether or not a given aspect of the child care experience had a particular effect.
The results of the study were pretty complex, but the two big findings that stood out:
An evaluation of the children in fifth grade showed that the children who had higher quality child care continued to show better vocabulary scores, a correlation that was seen previously from kindergarten to third grade. Vocabulary was assessed using the Picture Vocabulary subtest of the Woodcock-Johnson Psycho-Educational Battery -- Revised, which measures children's ability to name objects depicted in a series of pictures.
The researchers found that the correlation between high quality care and better vocabulary scores continued regardless of the amount of time the child had spent in child care or the type of care. The researchers wrote that this finding was consistent with other evidence indicating that children with greater early exposure to adult language were themselves more likely to score higher on measures of language development. However, child care quality was not associated with improved reading skills after 54 months of age.
The researchers also found that, as in the earlier grades, children with more experience in child care centers continued to show, through sixth grade, a greater frequency of what the researchers termed teacher-reported externalizing problem behavior. These behaviors were listed on The Child Behavior Checklist Teacher Report Form, which consisted of 100 problem behaviors.
So, this data indicates that quality daycare is associated with better language scores and more disruptive behavior. But, these results are very equivocal. First, the differences in language scores disappears after age 4 1/2. That can't be too important in the development of adult language.
Second, the relationship between disruptive behavior and time spent in daycare is very small. As they said in ScienceDaily:
The researchers emphasized that the children's behavior was within the normal range and were not considered clinically disordered.
It would not be possible to go into a classroom and with no additional information, pick out which children had been in center care, Dr. Belsky explained.
Significance versus Importance
So, what happened here? Very simply, there is a difference between statistical significance and importance. When we assess statistical significance, we estimate the likelihood that (in this case) a relationship is due to random chance. If the odds that the relationship is due to chance is less than 5%, we say that the relationship is "statistically significant."
One major factor affecting statistical significance is sample size. Smaller samples are more likely to have random errors affecting the results, so it takes stronger relationships to reach statistical significance. Large samples are less affected by random errors (they tend to cancel each other out), so weak relationships can reach statistical significance. In the NICHD study, they assessed 1,364 children. That's a very big sample. And they came up with very weak relationships. The relationships are not important.
Children in daycare might be slightly more verbal, but being in daycare won't make them grow up to be great orators. Children in daycare might be slightly more aggressive, but spending more time in daycare won't make them grow up to be criminals.