J Manipulative Physiol Ther. 2010 (Sep); 33 (7): 536–541 ~ FULL TEXT
Karin R. Wiberg, MSc, Jesper M.M. Wiberg, DC
Private Practice of Chiropractic,
OBJECTIVES: The aim of this study was to investigate if the outcome of excessively crying infants treated with chiropractic manipulation (1) was associated with age and/or (2), at least partially, can be explained by age according to the natural decline in crying.
METHODS: This was a retrospective evaluation of clinical records of 749 infants from a private Danish chiropractic practice. All of the infants were healthy, thriving infants born to term within the age of 0 to 3 months who fulfilled the diagnostic criteria for excessively crying infants (infantile colic), whose parents sought chiropractic treatment. The infants were treated using chiropractic management as decided by the treating doctor of chiropractic, and changes in crying based upon the parents' report were noted as improved, uncertain, or nonrecovered. Age predictor groups were cross-tabulated against the outcome variables, and difference between classification groups was tested with X2 tables and confidence intervals.
RESULTS: Slightly older age was found to be linked to excessively crying infants who experienced clinical improvement. However, no apparent link between the clinical effect of chiropractic treatment and a natural decline in crying was found for this group of infants.
CONCLUSION: The findings of this study do not support the assumption that effect of chiropractic treatment of infantile colic is a reflection of the normal cessation of this disorder.
From the Full-Text Article:
We found that improvement of crying was associated with slightly older age. This study appears to be the first attempt to clarify whether improvement from excessively crying infants behavior is caused by a natural crying pattern within the first 3 months of life. A natural crying pattern could not explain positively the outcome of excessively crying infants treated with chiropractic manipulation.
However, the following points must be considered when interpreting these findings. Ideally, a study should be designed as a prospective, RCT with a blinded observer, which was beyond our scope. On the other hand, to exclusively evaluate chiropractic treatment through use of the RCT would not only be restrictive; paradoxically, it might also generate invalid evidence because the RCT is divorced from the real world and not necessarily externally valid. 
Limitations of This Study
The choice of a retrospective design makes it impossible to assess whether the observed effect from chiropractic care was better than no treatment in the treatment for excessively crying infants. Also, this study design suffers from missing and inaccurate data because it is based on data collected from a routine practice setting.
This study has been adjusted to information available in existing clinical records instead of the reverse order, where the study is designed to collect all essential data. Furthermore, instead of diary recording, crying is based on the parents' subjective perception of the severity, which induces a risk of overestimating the duration of crying. 
Another weakness is the possibility of predominantly good results due to follow-up data being collected by the treating doctor of chiropractic. The patient may provide polite responses indicating improvement, and the clinician may interpret vague answers in a predominantly positive manner. Nevertheless, this is the typical clinical situation. The clinician assumes that the parent wants to stop their infant's excessive crying, and the parent assumes that the clinician can help with this. We expect that any exaggerated bias would be divided equally regarding age.
Potential of This Study
The bottom-up approach in retrospective study design is systematic and subjected to the same laws of statistics and epidemiology as the RCT. Furthermore, it has relevance to what actually happens in practice, and research evidence can easily be applied in the management of an individual patient, which is, after all, the ultimate goal of evidence-based practice. 
The infants in the control groups of the 2 published RCTs were subjected to the normal crying curve, which we in this study compared to positive outcome. The purpose of using this new approach is an attempt to determine whether there is a basis for further investigations of a possible relationship between an age-related natural crying pattern and improvement of excessively crying infants. Older infants tended to have better outcomes, but we did not find this tendency convincing enough to be a “natural resolution.” This study suggests that the clinical effect of chiropractic treatment for excessively crying infants may be affected but cannot be explained by the normal crying curve.
Data matched the clinical experience of good treatment effect independent of the infant's age. Slightly older age and longer duration of treatment were found to explain covariables linked to crying infants with improvement. No apparent link between the natural development of crying pattern and a positive outcome of chiropractic treatment was found. The findings of this study do not support the assumption that possible effect of chiropractic treatment of infantile colic is simply a reflection of the normal cessation of this disorder.
No apparent link between the clinical effect of chiropractic treatment and a natural crying pattern was found.
Slightly older age was found to be linked to crying infants with clinical improvement.