Do radiometric dating labs throw out 8 of 10 specimens, and are the samples blinded?

Question

I think that my question regarding KNM-ER 1470's and the change from previously believed "good dates" from methods other than the fossil record might be too broad. There is an aspect of the question that should be fairly easy to verify though. The creationist website claims:

No wonder radiometric dating labs require that all samples to be "dated" be identified as to their source in the Geological column! Approximately 8 out of 10 specimens ("dates") are discarded by radiometric dating labs because they are well out of range of age they "ought to be" given there source in the geological column. In their book POTASSIUM ARGON DATNG, PRINCIPLES, TECHNIQUES AND APPLICATIONS TO GEOCHRONOLOGY, Dalrymple and Lanphere sum up the whole circular process of radiometric dating:

"If the potassium-argon ages of a group of rocks agree with the stratigraphic sequence determined on the basis of physical relationships of fossil evidence, then the probability is good that radiometric ages are reliable..."(page 197)

Does Potassium Argon Dating, Principles, Techniques and Applications to Geochronology actually say if the quote above (page 197)? (is it taken out of context?)

Is it common practice for radiometric dating labs to require samples to identified as to their source in the Geological column? (Since it would be better science to blind the dating of the samples, by making sure that the source is unknown to the radiometric dating lab, why is this required?)

Answer 1

While I do not have a copy of Potassium Argon Dating, Principles, Techniques and Applications to Geochronology, a review of the text in Science Magazine reads:

"The book grew out of a pamphlet written in response to requests from a number of US Geological Survey geologists who wanted a better understanding of potassium-argon dating. The author attempted to preserve the simplicity of the original pamphlet while making the book more nearly complete and more useful. As they point out in their preface, the book is not intended to be a scholarly or comprehensive review of potassium-argon dating, but rather an introduction to the principles, techniques, and applications of the method. Nevertheless it succeeds in doing for potassium-argon dating what Willard Libby's book Radiocarbon Dating did for that radioactive clock; it provides a balanced and sufficiently comprehensive introduction to the subject for the nonspecialist user of the data."

It appears that this book is a very introductory text and the quote you present from the creationist website could very likely be a part of an introductory explanation of how K-Ar dates are done. This book is written for someone who has collected samples, sent them to a lab, received a report with the lab's measurements, and wants to interpret what they're seeing. Sometimes the measurement is different than you expected, and the quote is probably about trying to figure out where exactly the disconnect is - is it with your sampling? The sample prep? Your stratigraphy? The measurement itself? If you pull it out of context, it can look incriminating, especially to the lay person, but it seems like a pretty basic statement: "if the data you got from the lab agrees with the data you have, then the data from the lab is probably good".

As for the claim that eight out of ten samples are discarded, I couldn't find a citation for that in your source. It's certainly not the case in any literature I've read. In most publications, scientists list all of the measurements that were made, including those that they ultimately left out of their data analysis, with detailed explanations as to why any dates were discarded - usually with fancy language like "we believe that the specimens may have been contaminated with modern organics during the chemical pretreatment stage", meaning "I'm pretty sure I dropped a hair in there." I've never read a paper that threw out more than 2 or 3 out of 40 dates, and I'm pretty sure that if you were tossing 8 out of 10 dates your paper would never get published in the first place. If you can find a published paper with such a high rejection rate, I would be interested in reading it - something really weird must have been going on.

On the topic of blind studies: for the following, please keep in mind that my personal experience with radiometric dating is with radiocarbon, not K-Ar. However, the general principles involved in radiometric dating apply across methods.

Blind studies are employed to prevent bias in the results. When you're measuring something qualitative (non-numeric) like increased nausea or headache, a blind study is useful because prior knowledge could alter a person's self-assessment of these kinds of factors. When you're measuring something quantitative (numeric), like the ratio of K to Ar, prior knowledge would only matter if you're deliberately tampering with the measurements.

You'd only need to employ a blind study in doing quantitative research if there was reason to believe that the people making the measurements were not only untrustworthy, but also incredibly adept at altering or manufacturing records of the data collected by instruments. In radiocarbon dating, each sample is measured "for ~5 minutes, and the complete target wheel of 32 targets is rotated a total of 6 times, giving a total analysis time on each target of about 30 minutes." The instrument records the number of isotopes reaching the counting device every second and creates hundreds of megabytes of data per sample. "We routinely use the strip chart feature of the NEC analysis code (abc) to scan the cycle by cycle data records to investigate anomalous results." That is, we look at every single data point that the instrument generates and we keep that data on file, which is available on request.

Fudging that much data in a convincing way would take effort and a depth of expertise that makes me sleepy just thinking about it. More than that, just think of the conspiracy that would be involved - just considering radiocarbon there's well over 50 radiocarbon dating labs in the world, and each lab employs tens of people including undergraduates who have no stake in an old earth, and would absolutely love to be the person who broke this vast conspiracy like something out of a Nancy Drew novel. No scientist I know has the kind of spare time or vindictiveness necessary to pull off that kind of thing.

That doesn't mean we never do blind dates. One of the times that it's done blind is in laboratory intercomparisons. In radiocarbon, for example, there have been several International Radiocarbon Intercomparisons. The most recent intercomparison, VIRI (Fifth International Radiocarbon Intercomparison), took place in 2010. Scientists at the University of Glasgow selected 7 samples which were sent to over 50 radiocarbon dating laboratories without information regarding the ages of the samples. These intercomparisions are done to learn about the variability in the measurements between labs, and so that these samples may be later used as standards. Standards are samples which have been measured repeatedly, and have an age that is well agreed upon. When measuring unknown samples, standards are always measured as well, so that you can be confident that the measurement is accurate. That is, if you can reproduce the value of the standard, you can be sure that the measurement made on the unknown is reliable.

Blind studies are not useful in standard radiometric studies, and in fact, knowing the approximate age of a sample is important so you can select appropriate standards. "To obtain a radiocarbon age the sample activity or the 14C/12C ratio must be compared to a standard material of known age." When you are measuring samples, you tune your instrument to obtain the best precision in the sample's range. So if your standard is of similar age to your unknowns, you will be able to get good precision for both your standards and your unknowns. If the measurement deviates widely from the expected value, you can use the measurement on the standard to be sure that the difference between the measured value and the expected value isn't a laboratory error.

Answer 2

The book does in fact say that on page 197 (I own it). The full paragraph reads:

If the potassium-argon ages of a group of rocks agree with the stratigraphic sequence determined on the basis of physical relationships of fossil evidence, then the probability is good that radiometric ages are reliable. An example of this are the data in table 7-7. For single age measurements of units that either overlie or underlie beds containing fossils, the fossil evidence only places an upper or lower limit on the age determination.

The same section, titled "Evaluation of age data", also suggests duplicate measurements, preferably from different minerals.

All data that scientists produce are, and should be, carefully examined for consistency. Dalrymple and Lanphere also suggest that several samples should be measured to be more able to identify problematic samples. If anything, this is good scientific practice, rather than taking a single measurement at face value.

In my own experience (I run an Argon geochronology lab), the approximate age of a sample is useful input to select the correct standards. The reason is analogous to trying to measure millimeters with a yardstick. If you calibrate your machine to measure very large amounts of Argon and you chose small sample size consequently, to have less gas load on the detectors, a young sample will then yield very large errors. It is possible and usually yields accurate, but less precise results (meaning larger error margins). The reasons for this are detailed in McDougall and Harrison: "Geochronology and Thermochronology by the 40Ar/39Ar method". We geochronologists strive to provide users with the best results possible. The exact age is irrelevant for the measurement, but it is useful to know it within an order of magnitude (millions of years, hundreds of millions of years or billions of years, for example).

See here for some examples where a geochronologist suggests that we should throw out more dates ! Simply because according to him, some labs have less stringent statistical requirements to accept an age. Although this particular paper addresses Ar/Ar dating, typically K/Ar method provides less tools to check for internal consistency and alteration, although in most cases K/Ar dates and Ar/Ar dates will yield similar results.