Analysing Friedel averages and differences.

Various practical applications of the average (A) and difference (D) of Friedel opposites are described. Techniques based on the resonant-scattering contribution to Friedel differences are applied to see whether a crystal is centrosymmetric or not, and to determine the point group of the crystal. For the validation of a structural study, plots of A(obs) against A(model), and D(obs) against D(model) are used extensively. Moreover, it is useful to display both plots on the same graph. Intensity measurements on a crystal of NaClO(3) were made at three different speeds, with two different radiations and two different diffractometers, and treated with two different software packages and four different absorption corrections. The evaluation of these numerous data sets reveals underlying deficiencies. For comparison, plots of A(obs) against A(model), and D(obs) against D(model) are presented for two centrosymmetric crystals.

Ab initio reconstruction of difference densities by charge flipping.

The charge-flipping algorithm in its band-flipping variant is capable of ab initio reconstructions of scattering densities with positive and negative values. It is shown that the method can be applied to reconstructions of difference electron densities of superstructures, i.e. densities obtained as a difference between the true scattering density and the average density over two or more subcells of the true structure. The amplitudes of reflections lying on the reciprocal lattice of the subcell are not required for the procedure. A series of examples shows applications of the method to the solution of superstructures in periodic crystals or quasicrystals as well as the application to ab initio solution of modulation of an incommensurately modulated structure from satellite reflections only and solution of a structure from a crystal twinned by reticular pseudomerohedry. The method is especially suited for solving pseudosymmetry problems occurring frequently in superstructures.

Nonmedical costs of colorectal cancer screening using CT colonography.

PURPOSE: Nonmedical costs are important because they may influence screening uptake and the results of cost-effectiveness analyses of colorectal cancer (CRC) screening. Although the nonmedical costs of CRC screening are known for fecal occult blood testing, flexible sigmoidoscopy, and colonoscopy, they are unknown for CT colonography (CTC). The purpose of this study was to determine the nonmedical costs of CTC. METHODS: Four hundred eighty-one consecutive individuals presenting for CRC screening using CTC were approached at a radiology clinic in Calgary, Alberta, Canada, to participate in the study. Of these, 197 agreed to have a questionnaire mailed to them. Subjects completed the questionnaire, including items on time off work, both for the subject and any accompanying caregiver; travel details; and direct out-of-pocket expenses. Time costs were valued at government of Canada wage rates. Travel costs included estimated costs for travel by car and actual parking costs and taxi and public transportation fares. Car user's costs were calculated using a Canadian Automobile Association estimate of motoring costs per kilometer. Costs are reported in 2010 Canadian dollars. RESULTS: One hundred thirty-two subjects returned the questionnaire (mean age, 57 years; 65% men; 67% employed). Ten subjects traveled >200 km for their CTC. The nonmedical costs (subject with or without caregiver) for the 122 subjects who resided within 200 km of the clinic averaged $101. CONCLUSIONS: The nonmedical costs of CRC screening using CTC are significant but lower than those of colonoscopy. Patients should be aware of these differential costs, and they should be incorporated into economic analyses of CRC screening.