In parts 1 and 2 of this series, I discussed why posture matters to us as chiropractors from a scientific perspective. Part one discussed the very high level of "face validity" (the degree of obvious relationship between a conclusion and the premises used to reach that conclusion) of incorporating postural analysis of spine as compared to focusing solely on the dynamics of individual vertebral joints. In part two, we discussed the very high level of inter and intra examiner "reliability" of radiographic measurements used to quantify postural misalignment. This month I want to discuss the two related concepts of "accuracy" and "precision," to demonstrate the profound importance of postural chiropractic to working chiropractors in the field and the profession at large.
Accuracy vs. Precision
Both terms are often used interchangeably in common speech but, in fact, each has specific and unique meanings in science. The definitions of each term are relevant to what you do every day in your office. Let’s use a simple shooter's target as a quick analogy.
Accuracy: The target in Fig. 1 represents a cluster of four shots, all of which are placed very near to the bull's-eye, but not very close to one another. In this sense, the shooter is demonstrating a fairly high degree of "accuracy." That is, he/she is able to place all four shots fairly close to the bull's-eye, but the individual shots are not particularly close to one another. There is a lack of consistency in the pattern.
Precision: By contrast, the target in Fig. 2 shows that the shooter has placed all four shots very close to one another, demonstrating a high degree of "precision." The results of the shooter's efforts are highly consistent. Unfortunately, all of the shots have missed the bull's-eye by a wide margin.
In practice, we constantly need both accurate and precise information. We need to know what's actually wrong with our patient, guidance as to the proper course of treatment, and methods of reliably assessing the effectiveness of our treatment. I have already discussed the very high degree of inter and intra examiner reliability of various radiographic postural measurements in article 2 (Feb. 2010) of this series. The ultimate test of how much we as doctors should "rely" upon the results of any method of analysis requires us to understand how both "accuracy" and "precision" impact the end product of "reliability." My apologies for all the semantics here but, hopefully, I can demonstrate the relevance of this to all of us as practitioners.
In our target analogy above, the bull's-eye represents Truth. Any analytical procedure is then deemed "accurate" if it yields a truthful result. In other words, the procedure actually measures that which it purports to measure. Let's take a simple blood pressure measurement, a procedure which appears to have a high level of "face validity," as an example. If I take a patient's blood pressure and note an increase in the numbers on the cuff, it's quite likely the finding represents an actual increase in the patient's B.P. In this sense, the assessment of blood pressure appears to be a generally accurate, or truthful, procedure. That is, the test does what it is supposed to do...measure blood pressure.
Precision is another question entirely. For example, if I measure the pressure three times and get three widely differing values, it would appear that the information isn't particularly "precise" or reliable. By the same token, if I get the similar results three times in a row, then the procedure would appear to be fairly precise. It's easy to see, then, that reliability is intimately wedded to the inherent precision or consistency of a diagnostic procedure.In terms of how this relates to the analysis of postural imbalance using radiographic measurement, consider the following. Simple geometric measurement of spinal X-rays has repeatedly proven to be a remarkably "precise" method for measuring spinal displacement. The level of "precision" in measuring rotational spinal displacements is often within a degree or two. And linear displacements of the spine (such as forward head translation) can often be measured within a couple of millimeters. In a profession where two chiropractors can't even agree half the time as to which segment needs to be adjusted, that's pretty impressive.1
In study after study, radiographic posture analysis has proven far more precise and, therefore, reliable than almost all of the more common methods of chiropractic spinal analysis in day to day use. The above referenced study by French actually found that our more commonly used methods of analysis (leg length discrepancy, neurologic tests, motion palpation, static palpation, and orthopedic tests among others) only had fair to slight inter and intra examiner agreement. Speaking in terms of how and when chiropractic treatment should be implemented, the author concluded "these examination techniques alone should not be seen by practitioners to provide reliable information." Similarly, Troyanovich, et al., concluded that, "Most patient assessment methods used in chiropractic have poor or unknown reliability. The one possible exception to this rule is spinal displacement analysis performed on radiographs."2
And yet, many chiropractors fail to incorporate simple methods of radiographic measurement of postural misalignment. Despite what you may have been told, good old plane film X ray is one of the most accurate and precise diagnostic tools available to you. Certainly, it has some limitations and inherent weaknesses but, when it comes to structural analysis of the spine, nothing else is even close. And, let's face it folks, regardless of your personal technique or philosophy if, as a profession, we don't have accurate and precise methods for assessing the structure of the spine, then we have literally lost our professional legitimacy.
Dr. Mark Payne is president of Matlin Mfg., a manufacturer and distributor of postural rehab products since 1988. For a FREE, unabridged copy of this article or other information on postural chiropractic, please contact Matlin Mfg. Inc. at 1-334-448-1210 or on the web at www.MatlinMfg.com.
1. Reliability of chiropractic methods commonly used to detect manipulable lesions in patients with chronic low-back pain. French SD, Green S, Forbes A. J Manipulative Physiol Ther. 2000 May;23(4):231-8.
2. Chiropractic biophysics digitized radiographic mensuration analysis of the anteroposterior lumbopelvic view: a reliability study. Troyanovich SJ, Harrison SO, Harrison DD, Harrison DE, Payne MR, Janik TJ, Holland B. J Manipulative Physiol Ther. 1999 Jun;22(5):309-15.PMID: 10395433 [PubMed - indexed for MEDLINE]