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Therefore for k levels of a variable cheap 50mg danazol free shipping women's health workout abs, there will be k − 1 dummy variables buy danazol 100 mg line pregnancy x ray lead apron, for example buy 200 mg danazol with visa breast cancer awareness products, for a variable with three levels discount 50mg danazol mastercard breast cancer quilt pattern, two dummy variables will be created. It is helpful in interpreting the results if each dummy variable has a binary coding of 0 or 1. The variable parity1 with three levels from Chapter 5, that is parity coded as babies with 0, 1 or 2 or more siblings, can be recoded into dummy variables using Transform → Recode into Different Variables. Dummy variables are invalu- able for testing the effects of ordered groups that are likely to be different, for example, lung function in groups of non-smokers, ex-smokers and current smokers. It is essen- tial that dummy variables are used when groups are non-ordered; for example, when marital status is categorized as single, married or divorced. Related dummy variables must always be included in a model together because they cannot be treated independently. If one dummy variable is significant in the model and a related dummy variable is not, they must both be left in the model together. In the Model Summary table, the adjusted R square value shows that the addition of the dummy variables for parity improves the fit of the model only slightly from 0. In the Coefficients table, the P values for the unstandardized coefficients show that both dummy variables are significant predictors of weight with P values of 0. However, the low standardized coefficients and the small partial correlations in the Excluded Variables table show that the dummy variables contribute little to the model compared to length and gender. Using the values in the Coefficients table, the regression equation is now as follows: Weight =−4. The coefficients for the final two terms indicate that after adjusting for length and gender, babies with one sibling are on average 0. The previous regression model with one conti- nuous and two categorical variables, that is, length, gender and parity, can be further extended with the addition of second continuous explanatory variable, that is, head circumference. The final predictive equation could be used to generate normal values for term babies, to calculate z scores for babies’ weights, or to calculate per cent predicted weights. Correlation and regression 229 The regression model obtained previously can be built on to test the influence of the variable, head circumference. The model in which parity2 was included as a binary vari- able is used because including parity with three levels coded as dummy variables did not substantially improve the fit of the model. The Model Summary table shows that the adjusted R square increases slightly from 55. The Change Statistics indicates that the increase in R2 from Model 1 to Model 2 is significant. In the Coefficients table, all predictors are significant and the standardized coefficients show that length contributes to the model to a greater degree than head circumference, but that head circumference makes a larger contribution than gender or parity. This is expected because the initial Pearson’s correlations showed a significant association between length and head circumference with an r value of 0. As a result of the mutlicollinearity, the standard error for length has inflated from 0. Head circumference is expected to vary with length as a result of common factors that influence body size and growth. In this situation, head circumference should be classified as an alternative 230 Chapter 7 outcome rather than an independent explanatory variable because it is on the same developmental pathway as length. Each model building situation will be different but it is important that the relationships between the variables and the purpose of building the model are always carefully considered. If an interactive effect is present, the two lines would have different slopes and would cross over or intersect at some point. To test for the presence of an interaction, the two variables are multiplied to create a cross-product term, which is included in the multiple regression model. In the following equation, the fourth term represents an interaction between length and gender. Also, once again, coding of binary variables as 0 and 1 is helpful for interpreting interactions. When gender is coded as 1, the third term will add a fixed amount to the prediction of the outcome variable and the fourth interactive term will add an amount that increases as length increases, thereby causing the regression lines for each gender to increasingly diverge. The regression equation for a model with an interaction term would be as follows: Weight = a +(b1 × Length)+(b2 × Gender)+(b3 × Length × Gender) It is preferable to explore evidence that an interaction is present rather than testing for all possible interactions in the model. Testing for all interactions will almost certainly generate some spurious but significant P values. To avoid multicollinearity, the explanatory variables and their interaction can be centered before inclusion in the regression model,7 which will be discussed later in this chapter. The regression plots can then be inspected to assess whether there is a different linear relationship across the groups. When the values of the data points are a long way from zero, as in these plots, Correlation and regression 231 Gender: Male R2 Linear = 0. Correlation and regression 233 the y intercepts have no meaningful interpretation although they can indicate that the slopes are different. This similarity of slopes suggests that there is no impor- tant interaction between length and gender in predicting weight. The graphs can be repeated to investigate a possible interaction between head circumference and gender. If plotted on the same figure, the two regression lines would intersect at some point indicating an interaction between head circumference and gender. In practice, head circumference would be omitted from the model because of its collinearity with length but it is included in this model solely for demonstrating the effect of an interaction term. If an interaction term is included then both derivative variables, that is, head circumference and gender, must be retained in the model regardless of their statistical significance. Once an interaction is present, the coefficients for the derivative variables have no interpretation except that they form an integral part of the mathematical equation. The Coefficients table shows that inclusion of the interaction term inflates the standard error for head circumference from 0. These standard errors have inflated as a result of the collinearity with the interaction term and, as a result, the tolerance value in the Excluded Variables table is very low and unacceptable at 0. In addition, while the change in R square from Model 1 to Model 2 was significant, it is important to assess the clinical significance of this increase, in conjunction with a less precise model. This example highlights the trade-off between building a stable predictive model and deriving an equation that describes an interaction between variables. Multicollinearity caused by interactions can be removed by centering12 which is described later in this chapter. Correlation and regression 235 The final model with all variables and the interaction term included could be consid- ered to be over-fitted. By including variables that explain little additional variation and by including the interaction term, the model not only becomes complex but the preci- sion around the estimates is sacrificed and the regression assumptions of independence are violated. Head circumference should be omitted because of its relation with length and because it explains only a small additional amount of variation in weight.
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These are: (1) salt⎯50% caries reductions in Switzerland and Hungary; (2) milk⎯15-65% caries reductions; (3) mineral Water⎯46% caries reductions in Bulgaria danazol 100mg overnight delivery menopause type 7. Are we therefore receiving more than the optimum daily amount of fluoride and therefore at increased risk of fluorosis? Mineral waters are used extensively as the main source of household drinking water danazol 100mg low cost menstruation. In addition some baby milk formulas have high amounts of fluoride themselves cheap danazol 100 mg menstrual insomnia, and if made up with a high fluoride bottled water the infant may be at increased risk of developing dental fluorosis generic danazol 50 mg fast delivery breast cancer walks 2014. The maxillary permanent central incisors are most susceptible to fluorosis at about 2 years of age. The same applies to foods that are processed and canned or packaged in plants using fluoridated water. Toothpastes A dramatic decrease in worldwide caries levels has been seen since their introduction in the early 1970s. Child formulations contain up to 550 ppm fluoride to limit fluoride ingestion and therefore reduce the risk of fluorosis. A systematic review of low fluoride toothpastes showed a reduced efficacy of 250 ppm fluoride in comparison to 1000 ppm fluoride. Therefore, it is advisable to recommend toothpastes for children containing at least 500 ppm fluoride to ensure caries preventive efficacy. It is sometimes difficult to decide which concentration of fluoride toothpaste is to be recommended to parents for their children. However, if a young child under 6 years presents with caries, a fluoride toothpaste of at least 1000 ppm is indicated as these have been proven to be more efficacious for caries prevention. Fluoride gels These can be applied in trays or by brush and 26% caries reductions have been reported. There is a risk of toxicity with the high fluoride containing gels and the following safety recommendations should be followed: (1) no more than 2 ml per tray; (2) sit patient upright with head inclined forward; (3) use a saliva ejector; (4) instruct the patient to spit out for 30 s after the procedure (usually 4 mins but newer types are for 1 min). Home use gels contain 1000-5000 ppm fluoride for use by patients at home at bedtime in addition to toothbrushing. It is best to advise patients to use their fluoride rinses at a different time to toothbrushing so that the number of fluoride exposures increases. The effect of toothbrushing and rinsing with fluoride has been shown to be additive. All orthodontic patients should be using a daily fluoride rinse to minimize the risk of demineralization and white spot lesions. Children under the age of 6 years should not be recommended to use fluoride mouth rinses due to the increased risk of swallowing the product. Varnishes Duraphat 5% by wt fluoride = 22,600 ppm fluoride is the main fluoride varnish. It is supplied in a small tube, but used lavishly by most dentists as if it were toothpaste. It should be used sparingly with a cotton bud, a small pea-size amount is sufficient for a full mouth application in children up to 6 years. Slow-release fluoride devices Many dental materials like amalgam, composites, cements, acrylics, and fissure sealants have had fluoride added, but the fluoride release was either short term or the properties of the materials were adversely affected, to make them of any use to provide a long-term source of intraoral fluoride. Glass ionomer cements are a group of materials that have fluoride, but long-term release is debatable. That is when the fluoride is released from the material it later takes up fluoride from other dental products that are used by the patient, for example, fluoride toothpaste or mouth rinse, and this fluoride is released at a later time. The objective is to develop an intraoral device that will release a constant supply of fluoride over a period of at least a year. Studies in Leeds demonstrated that there were 67% fewer new carious teeth and 76% fewer new carious surfaces in high caries-risk children after 2 years in a clinical caries trial for children with the fluoride devices in comparison to the control group with placebo devices. There were 55% fewer new occlusal fissure carious cavities showing that occlusal surfaces were also protected by the fluoride released from the devices. The fluoride glass devices have been patented and commercial development is now under progress. The provision of fluoride for each individual must be tailor-made to suit varying social and working circumstances. Slow-release fluoride devices seem ideal for targeting the high caries- risk groups who are notoriously bad dental attenders with very poor oral hygiene and motivation. This is a very promising development with application for use in numerous high-risk groups including the medically compromised. Deciding which fluoride preparation to use for differing clinical situations: This will depend on: (1) Which groups of children? In addition, the expected patient/parent motivation and compliance is very important in deciding what to use. Initially developed to prevent caries their use has been developed further and they now have a place in the treatment of caries. The decline in caries observed in industrialized countries over recent decades has affected all tooth surfaces but has been greatest on smooth surfaces. Therefore the pit and fissured surfaces, particularly of the molars have the greatest disease susceptibility. This means that the potential benefits of effectively used sealants continue to increase. The technique for placement of sealants is relatively simple but is technique sensitive. Salivary contamination of as little as half a second can affect the bond and therefore the retention of the sealant. Current resin materials are either autopolymerizing or photo-initiated, and most operators prefer the advantages of demand set offered by photo-initiation. Although there are theoretical advantages to chemically cured materials in terms of retention, as these materials have longer resin tags extending into the etched surface. Filled and unfilled resins are available, the filled materials being produced to provide greater wear resistance. However, this is not clinically relevant and clinical trials demonstrate superior efficacy for unfilled materials. Irrespective of the presence of fillers some materials are opaque or tinted to aid evaluation. This is an advantage but means the clinician is unable to view the enamel surface to assist with caries detection and to detect the presence of restorations such as sealant restorations. Key Points Fissure sealing technique • Prophylaxis before etching does not enhance retention but is advisable if abundant plaque is present. A dry brush should be used rather than paste as these are retained in the depths of the fissures preventing penetration of the resin. Operator and assistant must act as a team as it is impossible for single operators to apply sealant effectively. The vast majority of trials have demonstrated cotton wool and suction to be an effective means of isolation. Rubber dam is advocated by some because of the superior isolation offered by this material.
