Boushey et al. (2017) distinguish image-assisted methods, in which photographs support a conventional record or recall, from image-based methods, in which the photographs are the primary record. The Remote Food Photography Method is the best documented of the latter. Users send images of food selection and leftovers to a server, and analysts estimate intake by comparison with reference images of known portions (Martin et al., 2009). It has been compared with weighed intake and with doubly labelled water in adults and found to underestimate energy intake by a modest margin (Martin et al., 2012). The approach grew out of digital photography of cafeteria trays, which was shown to agree closely with weighing (Williamson et al., 2003).

A weighed food record, in which every item is weighed before eating and leftovers are weighed afterwards, has traditionally been treated as the most precise practical measure of intake over the recorded days, and other methods have been judged against it (Bingham et al., 1994). It is not a true gold standard. The act of weighing changes what people eat, and comparisons with doubly labelled water show that self-reported energy intake, however recorded, tends to fall below expenditure (Schoeller, 1995). Standard texts therefore speak of relative validity when one self-report method is compared with another (Willett, 2013). The protocol below accepts that framing and takes steps to limit its weaknesses.

A high correlation between two methods shows that they rank people similarly, not that they give the same values. Bland and Altman (1986) proposed plotting the difference between methods against their mean and reporting the mean difference, or bias, with limits of agreement within which most differences fall. Whether those limits are acceptable is a judgement about the use to which the measurement will be put, and has to be made before the data are seen. A review of dietary validation practice recommends applying several statistics together, because each reflects a different facet of validity (Lombard et al., 2015).

The investigator first states what the photo diary will be used for: ranking individuals in an epidemiological study, estimating group mean intake, or monitoring a patient's energy and protein intake in practice. Each implies a different standard. The nutrients and food groups of interest are listed, usually energy, protein and a small number of others relevant to ageing. For each, the largest bias and the widest limits of agreement that would still allow the intended use are written into the protocol, with the reasoning, before any recording begins.

The application should require as few actions as possible: open, photograph, confirm. Large buttons, high contrast and spoken prompts help users with poor sight or unsteady hands. A reference card of known size placed beside the plate gives the analyst a scale. Training is given in person, with practice on real meals, and covers the angle and distance of the photograph, the need for a second image of leftovers, and what to do about second helpings, drinks and snacks taken away from the table. Inclusion criteria should be set with the target population in mind. If people with mild cognitive impairment are to be included, a brief screening instrument such as the Mini-Mental State Examination (Folstein et al., 1975) allows results to be examined by level of cognition.

Validity is assessed on days when both methods are applied to the same intake. Two arrangements are possible. In the first, the participant both weighs and photographs every item. This is simple but makes the errors of the two methods dependent, since a forgotten snack is missing from both, and it doubles the burden. In the second, a researcher weighs foods and leftovers unobtrusively while the participant only photographs, which is feasible where meals are served, as in day centres, residential care or a research kitchen. The second arrangement gives a more independent reference and is preferred where the setting allows. Recording should cover several days, including a weekend day, and should not be consecutive if fatigue is a concern.

Sample size deserves an explicit justification. Guidance written for the validation of dietary questionnaires holds that studies with fewer than about fifty participants give estimates too imprecise to interpret and that a hundred or more is desirable (Cade et al., 2002; Willett, 2013). The precision of the limits of agreement, expressed as confidence intervals, should be the basis of the calculation.

Images are coded by trained analysts who have no access to the weighed data. Each food is identified, matched to an entry in the food composition database, and its portion estimated against reference images. The same database and the same recipes for mixed dishes are used for both methods, so that differences reflect the recording method and not the nutrient values. A subset of images is coded independently by a second analyst, and agreement between analysts is reported with an intraclass correlation coefficient chosen and described according to accepted guidance (Koo and Li, 2016). Images that cannot be coded, and the reasons, are counted and reported.

Table 1 lists the statistics proposed and what each contributes. The Bland-Altman analysis is central: it shows the average bias, the spread of individual differences, and whether the difference grows with the size of intake. Correlation and cross-classification into thirds or quarters of intake address ranking. A paired comparison of means addresses group-level agreement. Results are judged against the limits fixed in Step 1. Energy intake from both methods can also be compared with estimated requirements using the Goldberg approach to gauge the plausibility of the reference itself (Black, 2000).

A method that agrees with the reference but that older users abandon after two days is of little use. Completion is therefore measured directly: the proportion of eating occasions with both a before and an after image, and the number of days completed. Perceived usability can be recorded with the System Usability Scale (Brooke, 1996), and short interviews identify the points at which users struggled. Help given by relatives or carers is recorded, since a diary that works only with assistance is a different instrument from one used alone.

The report states the acceptance limits, the results for each nutrient, and a plain conclusion on whether the diary met them. Reporting should follow the STROBE-nut extension for nutritional epidemiology, which lists the details of dietary methods a reader needs (Lachat et al., 2016). Results for subgroups defined in advance, for example by age band, sex or cognitive score, are given with appropriate caution about their precision.