Correlation coefficients on the order of 0.5 to 0.7 appear to be typical for the reproducibility of nutrient intake . Cullen et al. (r = 0.21-0.72)  and Teiber et al. (r = 0.42-0.74)  reported reproducibility of FFQs for children. In our study, the correlation coefficients between the first and second FFQs were higher (r = 0.76 in CFFQ, r = 0.73 in AFFQ) than those in their reports.
We had assumed that the CFFQ would be a more suitable method for assessing children's diets than the AFFQ. The median of the adjusted values between the WDRs and CFFQ were r = 0.39 in the YC group and r = 0.34 in the AD group. The value between the WDRs and AFFQ were r = 0.30 and r = 0.29, respectively. The same moderate correlations were reported by Rockett et al. (r = 0.21-0.58) , Marshall et al. (r = 0.20-0.52) , and Blum et al. (r = 0.26-0.63) . When we classified the energy and nutrients by their correlation coefficients in adjusted values into 3 categories, for the YC group, the number of nutrients classified as medium and high was 28 in the CFFQ and 20 in the AFFQ. For the AD group, the number was 23 and 18, respectively. Comparing the CFFQ and AFFQ, the CFFQ was more suitable than the AFFQ in both the YC and AD group.
In our study, we found a small difference between the YC and AD groups in terms of the CFFQ or AFFQ.Cullen et al. compared the validity of the FFQ between a group of 10 to 12-year-old children and 13 to 17-year-old adolescents . They reported that the validity was higher in the 13 to 17-year-old group, probably because knowledge about foods or dishes was greater among the older subjects. However, we obtained higher median adjusted values between the WDRs and CFFQ or the WDRs and AFFQ in the YC group than in the AD group. The reason for such differences might have been because the mother completed both the WDRs and FFQs for the subjects in the YC group, while in the AD group, although FFQs were completed by the subjects, the WDRs were completed by either the subjects or their mothers. Because we considered that the subjects in the AD group had more opportunities to eat foods and snacks unknown to their guardians/parents than those in the YC group, we asked the subjects in the AD group to complete the FFQs. However, it might be possible that they had less knowledge about intake frequencies and the portion sizes of the dishes or foods than the mothers in the YC group. To maintain the quality of the data, the registered dieticians asked to the subjects to clarify any unclear answers.
We obtained different correlation coefficients for the same nutrients from the CFFQ and AFFQ, including sodium in the YC group (r = 0.17 in the CFFQ and r = 0.41 in the AFFQ). It is necessary to develop specific FFQs for the group being studied (to accurately reflect socioeconomic, cultural, and seasonal differences). The AFFQ was developed based on the dietary data of adults who lived in several different areas of Japan. Therefore, the AFFQ might have sufficiently covered the dishes and individual foods consumed by subjects in this study. However, the AFFQ was found to have lower correlation coefficients of total fat and fatty acids than the CFFQ, especially in the YC group. It may be necessary to modify a food list in the AFFQ to add foods influencing fat that are regularly eaten by children, and are familiar to children, such as snacks.
Generally, a FFQ originally developed for adults is applied for the assessment of dietary intake in children, however it has been reported that when the FFQ for adults is used, the dietary intake of children is overestimated [5–7]. Fumagalli et al.  studied the validity of a FFQ for 5 to 10 year old children, and Wilson  studied the validity of a FFQ for 4 to 9 year old children. These authors compared the improved FFQs for adults with 3-day WDRs. They reported that energy and nutrient intakes in the FFQs were overestimated. In our study, we compared the differences in nutrient intake between the WDRs and FFQs. The numbers of over/underestimated nutrients were lower from the CFFQ in the YC group and from the AFFQ in the AD group. The portion size of a FFQ should be suitable for the subjects. Because steamed rice is a particularly important energy source in Japan, it is important to determine the intake of steamed rice as accurately as possible, and three sizes of rice bowls (large, medium, and small) were included in the questionnaire in both the CFFQ and AFFQ (150 g, 120 g, and 80 g in the CFFQ and 200 g, 150 g and 100 g in the AFFQ). The portion size of the CFFQ is appropriate for 3 to 11 year old children, because the CFFQ was developed for this age-group children. Since the AFFQ was developed for adults, the portion size of the AFFQ might be more suitable for the subjects in the AD group who were bigger than those in the YC group.
The values of sodium intake in the CFFQ were underestimated compared to the WDR. In contrast, the values in the AFFQ were overestimated in both the YC and AD groups. The systematic errors were found. For example, when we collected dietary data from children for developing the CFFQ, the subjects' parents/guardians freely decided the day for the WRD . Although we asked subject's parents/guardians to have the children eat habitual diets on the day they selected for the WRD, it was possible that a special day (e.g. a holiday or other celebration where special foods are eaten) may have been selected. Therefore, the food list of the CFFQ might have posed the systematic errors.
In order to assess habitual intake, dietary surveys need to be conducted for several days . However, the WDR is considered a burden some for subjects. In addition, it has been reported that long-term dietary surveys are not always accurate . Therefore, we decided that the mean intake recorded in four WDRs, which were conducted on different days of the week, represented children's habitual dietary intake for one month.
In this study, the two types of FFQs, (i.e. CFFQ and AFFQ) were answered at the same time. We eliminated the any bias which may be caused by administration of the CFFQ and AFFQ by assuring that the order in which each FFQ was answered was randomly determined.
We were not able to assemble a large number of subjects of each sex and age. Therefore, we were not able to calculate the correlation coefficients by sex and age. For the growing children, it is thought that the amount and quality of the dietary intake differs by sex and age. Thus, we have adjusted for sex and age, in addition to energy, for correlation coefficients between the WDR and second FFQ.
We were not able to select subject candidates by random sampling because we had asked the teachers at each school to select participants. The four-day WDRs and 4 FFQs were a heavy burden for the subjects and their parents/guardians. To conduct a dietary survey at the kindergarten, elementary school, and secondary school, the cooperation of the teacher of each school was necessary to ensure that the parents/guardians understood the study. Therefore, the subjects and their mothers who participated in this study might have had more interest in their diets than the general population. In addition, because we specified the days of the dietary survey for subjects and their mothers, it was possible that the subjects may have eaten special meals on those days. Therefore, when we explained the study method to them, we asked them to eat habitual diets on the days of the dietary survey.
The CFFQ does not encompass the seasonal variation in the 75 food items, because when we developed the CFFQ from WDRs collected during only one season, between May and July . To address this potential short-coming, we decided to add seasonal fruits for this study. However, this study was conducted in the only one season. We need the further study to assess the seasonal intakes.
To our knowledge, this is the first study to examine the reproducibility and validity of an FFQ including questions about both individual food items and mixed dishes in 3 to 16 year old Japanese children. The CFFQ might be a useful tool for assessing the habitual dietary intake of 3 to 11 year old children for epidemiologic studies in Japan. In order to more accurately estimate the dietary intake of 12 to 16 year old children, it is necessary to develop a new FFQ or further modify the existing AFFQ or CFFQ.